Largest average sized tank for that space is a 29 gal
(30 1/4 x 12 1/2 x 18 3/4) but you could custom build one to match better. I assume the 35" long is due to how the stand is setup but you could use two tanks and have them connected to increase your water volume and/or for a fuge.
The 20" high is going to limit your skimmer selection. Most 125 gal
stands are 30" high so I'm confused why you're limited to 20".