You need to get whatever PVC is recommended for the bulkhead you get.
Also bare in mind drilling a tank is dangerous and best done outside with someone helping you. Safety glasses are a MUST, DONT even think about doing it without them.
I would also highly recommend, plumbers putty to make a reservoir around the drill bit with a constant supply of water to wash away the glass fragments. Slow and steady pressure, I have seen someone drilling a tank, only to crack the glass as the drill hit the side once the bit penetrated the other side.
In terms of the bulkhead size, you need to work out what flow rate you need and what your return pump is capable of, too much flow out the tank into the sump, without an adequate return pump and you will flood the floor.
I would look at doing 4 holes, 2 outflows perhaps in either corner and then two return holes towards the top. Here
is a link to an article on sumps, also a link
to an external site which offers extremely good advice on sumps.