Go as big as you can fit in the stand if you are going to be setting up a fuge. Otherwise, if you are just using to hide equipment (skimmer, heater, etc.), just big enough to fit the equipment and a return pump (if submersible).
Leave room for expanding amounts of supplies you may want to put in the stand too.
Personally, I am setting up my 90 now and have a 40 gallon breeder that I will be using as a sump.
**I am assuming you are putting the sump in the stand and not having it separate (in a basement, fishroom, closet, etc.)