If you read the literature, the wider tanks are recommended to encourage more gas exchange at the surface to ensure maximum oxygen levels. Tall narrow tanks look great for the viewer, but are less healthy for the inmates. I don't think you would want to go with a tank that is higher than it is wide, i.e. width = height, and length whatever your spouse lets you get away with.
Other concerns have already been mentioned: deeper tanks need stronger lighting (=$$$ in purchase and maintenance costs), bigger or more active fish need longer tanks, etc.
Personally, I can't stand low tanks. Everything looks like it's trying to survive in a puddle. Minimum height for me would be 30", but that's just my personal preference. YMMV
djm