Awesome that your building a tank. I love those domensions. I too hate tall tanks, your gonna love the width/depth it really changes the entire viewing experience. I think you would be fine with 20 inches.
You going with glass or acrylic ? Dont skimp on the stand. Mine is 27 inches tall (i think) and i wish it were a little taller, makes messing around in the sump a little difficult.
the stand I just built for my 100 gallon is 40" tall I figured all my fish are bottom dwellers so the higher the better now I won't have to bend so far to see them