Now, regarding the question...are you actually talking about building the sump tank from plate glass or acrylic or converting a pre-made tank to use as a sump?
Either way, figure out what dimensions will fit under your tank (or wherever you will put the sump). Test it out with a piece of cardboard the size of the proposed tank base...sometimes, the width of doors and bracing in the back can make for some headaches.
Decide if you will use an in-sump or external return pump and plan the space and connections accordingly. If external, you'll need a bulkhead of adequate size installed.
Go with the largest footprint that you can use (based on the above) but not necessarily the tallest. Remember that you'll want to get things in and out of that sump.
Also, if you plan to put the sump into the cabinet make sure that the bottom is reinforced to handle it.