Size of space where you will place the tank, which is also a function of the room or location in the house where you will keep it. You would also need to be mindful whether there will be a lot of movement around the tank, if it is on the way, if it is in a place where kid or pets can reach easily, and therefore the shape and tank measurements would need to be modified to account for this.
Number and size of intended inhabitants (including plants). As much as possible, you would need to account for the growth of your fish and their adult sizes and requirement of hiding space etc
Intended layout – remember that substrate for plants, and rocks and wood and back dropetc can take up considerable space in the tank. Henceit’s good to have in mind what the final tank will look like when you plan for tank capacity.