I'll offer this advice... I have a 90 and wish I bought a 120 (48x24). If you have the room I would go 24 inches deep. Much more depth and room for aquascaping.
I have to disagree. My advice would be to start small. I'm not saying to start with a 4 gallon pico, but there are some good AIO that come with everything you need to get started.
When I started (18 months ago) every one said go as big as you can. I bought a 90 gallon, stand, sump, lights, etc... $3000 and 18 months later I'm still spending on gadgets.
I'm still being told "your tank isn't big enough for that fish", and "your lights won't support those coral". So now I have a 4' tank with a few 2" fish?
I just bought a new 24 gallon Nano Cube with lights, hood, stand, pump and filters/media for $220. I'll need to add a few hundred in equipment (skimmer, cir. pump, ATO etc...) and I can put almost every fish I have in my 90 gallon in it.
Everyone will say a small tank is more maintenance, but I don't think so. I'm doing weekly to by-weekly water changes on my 90 and I'm positive the water change on the nano will be easier.
Also, the savings is not just in hardware. You save on salt, water, water change equipment etc...
I'm keeping my 90, gunna make it a fowlr, but I wish I would have started small.