There is the practical aspects like reach height + lighting, but consider the aesthetics of looking at the end. There isn't a right answer, but a personal preference. I've learned that I really don't like looking at cubes (so 24x24) and would prefer something more like (24x18) or (24x30). A long wide shallow peninsula is perhaps my favorite tank, but you might want more volume than 18 or 20" height. You may decide you really like cube and would rather do 20x20 than 24x20. See if you can sketch out a few online.
A key moment for me was seeing that Max E-260 at Manhattan Aquariums vs Reefer 250. I realized the lack of a center overflow and the extra height on the 260 (22"h with 18"depth) looked much nicer to me than the 250 with it's square end (20x20).
EDIT: Remember the width x length give you precious cabinet space, the height doesn't help. Having 20" or 24" depth is nice for cabinet + sump compared to 18"
EDIT2: If you're doing custom, I went with Coast to Coast. They were an absolute pleasure to work with, tank should be delivered this week.
EDIT3: Great view