There is no recommended size. The larger, the better is the best advice. You should be able to fit a 20 long under your stand.
The sump will hold a protein skimmer, and possibly a refugium. 2 excellent ways to remove nutrients. A wet/dry will provide more surface area for aerobic bacteria, which will do everything in the nitrate cycle BUT remove the nitrates themselves. So, for a fish only, this would be fine, but if you have a reef, and low nitrates are crucial, I would go with a sump.