I had reefmaina.net build a sump for my 90 that barely fits in my stand. Probably only about 1/2 an inch clearence on any side. The sump refugium design is the clasic refug on right skimmer on left return in the middle. I will hold about 15-20 gallons when running and more like 40 for overflow.
Are you going to have one custom built? Going to build it yourself? If you have visio I would be happy to send you my design. I got most of my ideas from http://www.melevsreef.com/allmysumps.html Marc builds sumps as well but I would recommend local building as shipping is questionable and very expensive.
As big as you can fit. What you want to do is sit down and figure out EVERYTHING equipment wise you need both going in sump and out of sump. Then build a pretend stand whether it be on paper or the actual stand itself. All of your equipment in relation to the space you have to work with will dictate how big you r sump can be.
If you have the luxury of being able to put the sump in another room, i say go as big as you can.
A 90 with a 90+ gallon remote sump makes for super ease of water changes(you can easily get away with it once a month and youre most like able to do it in a place with lots of room..).....and it makes for more stable water conditions.......
ive been toying with the idea of going with 250 gallon sump on my 90.............
Bigger sump gives you more water volume which increases your stability and dilutes out all the bad stuff that may be in your water. Let's say your tank produces one pound of waste a day (hypothetical, to show a point), well it's a lot better to dilute that waste in 200 gallons of water versus 100 gallons of water. As for your originial question, it depends on if you want a premade glass aquarium as your sump or just any container. If going with a glass sump, I would go with a 30 gal standard (36X12X16) or a 45 gal standard (36X12X24) - they would give you the volume that you want and a little extra room in the base of the stand. If just any container will do, get the biggest rubbermaid you can fit in the stand.