return pump size?

[mycat99]

i am setting up a 150 gallon tank with a 33 gallon sump .the tank is drilled for 1-1/2 inch overflow with 2 1 inch returns .there is about 4 feet from sump to top of tank .wondeering what size return pump i would need ?do i look at return pump size from head ?also what happen if you get too big of return pump?