If the bulkhead on the bottom of the overflow is 1", which is what it should be, you want at least a 1" piece of pool hose connected on the bottom of it. You will probably have to glue a 1" piece of pvc
into the bottom of the bulkhead so you can attach the pool hose. You will need a hose clamp for that. Just wait until you get the overflow box and go from there.
As for return line- you want the line to match your pumps outgoing end.
For instance, a Quiet One 3000 has a 1" exhaust port, and a Danner Magdrive 9.5 has a 3/4" outlet.
You are going to want to purchase a balll valve for whichever pump you choose so you can throttle it down to the right gph
. ( always connect the ball valve to the exhaust end, NOT the intake) You want to choose a matching pump, but you have to also take into consideration head loss.