Size of tank, size of sump, size of return pump, wants, needs....etc...

Generally 1" bulkhead and 3/4" return for most tanks up to 90g. You'll see either larger drains and returns or dual overflows for larger tanks. Keep in mind that the hole will be larger than the bulkhead used. This is generally set by what brand of bulkhead you choose. Most are a standard size for aquarium use, some like Hayward are larger holes for the same size bulkhead.

Check with Ace in SA as he does some tank drilling and might know what size hole for what size bulkhead.

Standard PVC glue is all you need for your project. You could get "hot" glue (AKA wet glue) but you don't need to waste the money on it.