The pig must fit loosely in the launcher/receiver so the barrel of the launcher/receiver must have a greater diameter than the pipe itself. When the pig is received the fluid has to pass around the pig in the barrel, so it would be a good idea to give the barrel a cross sectional area of about twice that of the linepipe, In your case say 28 inches. That will ensure that the velocity of the fluid passing around the pig is similar to the flow rate in the pipe when one or more pigs are in the barrel. You can make the barrel diameter smaller, however I would size it so that the flow velocity around the pig is less than 20m/s when flowing gas and 4m/s when flowing liquid. Be generous with barrel length. It is common for the barrel to hold a batch of pigs (say four or six pigs). Also, if you are going to use an intelligent pig on this line you will be surprised how long they can be. Find out what pigs you are going to use and make sure the barrel has adequate length.
The reducer between the line and the barrel is not a trivial issue. I would use an eccentric reducer with the flat side on the bottom, at least on the launcher. A pig is easier to launch when the barrel and the pipeline bottoms are on the same level. If you use a concentric reducer it is difficult to raise the pig up at the end of the barrel and stick its nose into the line. Also, it sits there at and angle and sometimes jambs.
If your launcher already has a concentric reducer then design a sliding carriage which you can use to locate the pig on the centre-line and slide it down the barrel into the mouth of the pipeline. This arrangement will also require a rammer to ram the pig into the mouth of the pipeline so that the carriage can be withdrawn. A sturdy wooden pole will do. Do not use aluminium in the sliding carriage and rammer because aluminium creates sparks when struck on rust. The barrel must be fitted with a closure and it is normal to install a quick opening closure. These are expensive pieces of equipment but make opening the barrel an easy task. I personally like the GD closure, most probably the most expensive around. The closure mechanism MUST have a means of ensuring that it can not be opened when their is pressure in the barrel. This normally takes the form of a small valve that accesses the barrel through the closure which must be open before the closure can be opened. It would be very dangerous indeed to open a closure when their is even a small pressure in the barrel so you MUST have a warning device.
The launcher/receiver must have an isolation valve between the barrel and the pipeline. That valve MUST be a full bore valve to allow the pig to pass unrestricted into or out of the barrel. I am sure all of the valves on your pipeline will be full bore so that pigs can pass unrestricted through the pipeline. Also, it is very important that this valve have high quality sealing. It must be either a soft seated ball valve or a ball valve with metal to metal seating where the seats are ground and lapped-in to effect excellent sealing. This valve is the only barrier you have between the line contents and yourself when the closure is open, and if it leaks you will not only have fluid all over the show, but you may also find it impossible to open or close the closure because you can not get rid of the pressure or the flow.
For the same reason, the barrel must have a good drain valve if it is a liquid system, or a good vent valve if it is a gas system. You need those to depressurise the barrel before opening it. Be generous with the sizing. It is very common for the barrel isolation valves to leak with time. Your colleagues may well need the good draining capability twenty years down the track.
Each barrel is attached to the pipeline through a TEE. The line runs straight through the tee into the barrel, and the fluid is diverted around the barrel through the branch of the tee. Generally the tee is an equal tee however it is also common for the branch to reduce a size or two. The problem with this tee is that when a pig passing through it it could attempt to travel down the branch and not the straight run, so it could jamb in the tee. To prevent that ALL TEE's on a pipeline with a branch size more than say 40% of the pipe diameter are barred. Barring means that steel bars are welded across the branch of the tee flush with the inside of the pipe to guide the pig through the tee and to prevent it moving into the branch. There are a number of ways of doing this. The simplest and cheapest is to weld bars in there. That must be done with care to ensure that the welding does not affect the metallurgy of the tee or introduce cracks or imperfections. You may even have to heat treat the tee after welding. Another way is to weld a short stub of pipe onto the branch and to design plates that weld into that sub that protrude out into the tee where they act as bars. This whole assembly is heat treated after manufacture and then simply welded on the branch of the tee. Another way to do it is not to cut the line at the tee at all but to install a tee that wraps around the pipe. The branch of the tee is accessed by cutting slots in the pipe in the branch area.
The tee must be located on the pipeline side of the barrel isolation valve and if it has a wall thickness greater than the linepipe, the inside diameter must be transitioned at a slope of about 1 in 4 down to the linepipe inside diameter. If this were not done the ledge created between the two pipe sizes could jamb the pig.
It is common practice to install a pig signaller (called a pig sig) downstream of the barrel isolation valve and the tee. This means that on the launcher the pig sig sits downstream of the tee and one the receiver it sits on the piece of pipe between the isolation valve and the reducer into the barrel. They are placed at that location because both the tee and the valve are capable of jamming the pig, so if the pig sig activates there is absolutely no doubt that the pig is either on its way or safely in the receiver. Make sure you put a short length of pipe between the reducer and the barrel isolation valve. That pipe gives space for the nose of the pig in a launcher, and is a good place to locate the pig sig in a receiver.
The only other thing required is the bypass line around the barrel isolation valve. This is called the kicker line. It generally tees off from the branch line from the main tee I described above, and connects to the launcher or receiver barrel. You would not tee off the main line with the kicker line because that would require a second barred tee. It is not critical where it stabs into the barrel but it would seem a good idea to stab into the closure end of the launcher (to get behind a pig to push it) and into the reducer end of a receiver ( to push the pig to the closure end of the barrel). The kicker must have at least one isolation valve with the same excellent sealing capabilities as the barrel isolation valve for the same reasons as the barrel isolation valve. It is generally sized for a maximum flow velocity of say 4m/s for liquid and 20m/s for gas. Also, the kicker line valve is used to throttle the flow from full pipeline pressure to zero pressure. Use a valve (or a valve combination) that is good not only for sealing but also throttling. I like to use a high quality plug valve for this service. Put a few kinks in the kicker line for thermal expansion if the pipeline fluid is hot.
The only other major issues needing attention is the support of the launcher and receiver. Most are designed with two saddle supports. Design those as vessel supports to support the launcher or receiver. Also, keep thermal expansion in mind when tieing the supports to the foundations.