The two main concerns I would have with sump size on this tank are:
1) needs to be large enough to fit any pump(s), protein skimmers etc... that you plan to place inside the sump
2) as with any sump make sure it is large enough so that during a power outage your sump won't overflow.
Since you have a Hex tank, oxygen exchange might not be quite as good as a rectangular tank which will have more surface area so you'll need to make sure you have an adequately sized wet/dry system to improve your gas exchange depending on how heavily you are going to stock. Are you planning on making a wet/dry system on your own or will you buy a stock system? Building one on your own will allow you to customize your size (Just look through all the Rubbermaid products at Home Depot or Walmart and find something that fits.
