Also the size of the hole will not be goverened as much by the size of the return and drain lines but more the size of the return and drain line bulkheads. For example a 1" drain line could require a 1 3/4" hole to be drilled so you can install the properly sized bulkhead to seal the hole and provide a place to connect the pvc
Have a glass shop or your LFS
drill the tank. A glass shop will proably be cheaper. The drain lines and return lines will be connected to a sump that is below the tank. It is in the sump that your skimmer, heater and any other filtration you have ill be placed.