With the measurement notes you made while determining the aquarium location in hand, shop around for the size of tank you have figured out you have room for, along with the stand to display it on. If you are looking at buying a complete all-in-one aquarium/cabinet/stand/light hood/canopy combination, make sure you have ample room for this type of unit if you are limited on space.
If just measuring the total height of the tank and stand, it's a good idea to consider the extra space needed on top for adding a light hood/canopy in the future, if you are not purchasing one at this time.
What happens if you buy the tank and stand, then later buy a light hood and find you are short of space to put it on top of the aquarium?
One of the best ways to decide what appeals to you is to see what your LFS (Local Fish Shop) has on display. You can also view a lot of tanks with little effort in Tank Photos. Many of these photos include a description of the tank equipment.
A couple of hints about picking a tank size. You are going to have to think about your tank lighting, at this time. Most lighting comes in 12", 18", 24", 36", 48", 60" and 72" lengths. There are the Metal Halide (MH) and Power Compacts (PC) available, also. These lights pack a lot of wattage into a small fixture.
You might want to think about building your own tank. Putting a tank together is incredibly easy. Just have your local glazier cut your tank panels to your dimensions, sand and clean the edges and stick it together with silicone caulking.
Assembling almost any tank takes less than 10 minutes.
When it comes to tank costs, the taller the tank, the thicker (more $$) the glass must be. You can determine the glass thickness you will need from the DIY Custom Aquarium Glass Thickness Calculator. Instructions and plans for Building Your Own Glass Aquarium are also available in the DIY Aquarium Projects Index.
The information and instructions in DIY Custom Glass Aquarium will give you the resources needed to build an odd sized tank.
Now that you have decided on your tank size and location, you will need to find something to put it on or in.