20 Gallon long tanks give you the volume with less height and greater length. Size DOES matter: length gives you easier aquascaping and maintenance and in addition, shallower water depth allows for better light penetration.
As for an office tank, it can work. However, I've found that my tank can always use a little 'tweeking'. On the other hand, once the tank matures, it CAN go on autopilot. But...salt water tanks can surprise you. The problem is..if something goes wrong in a 75G office tank, probably no problem. If it happens in a 10 or 20G....NUKE CITY. SH