There are no real rules for the reefing hobby. What works for you might not work for me, or the next guy. If you want a recommendation for what size of sump to use under your 65 gal. tank, I would say as big as you can fit in your stand.
Don't forget to plan for upgrades you might do in the near future, as far as available space under your stand for calc. and phosban reactors, additional pumps, Q.T. tank, top off bucket or whatever else you think you might add to your system down the road.
There never seems to be enough room under the tank.