How much water will you need? The number of live stock and their water volume needs will guide the overall water volume and size of your system.
How much swimming room will each specimen need? Will you need space in between corals? Will crowding cause stress and/or conflicts?
Of course, you can find examples or public and private reef aquarium systems that packed so tightly the fix appear to have little room to even turn around.
Too small and you limit your choices AND your system could be sensitive to repaid water quality changes.
Too big and your system is expensive to purchase and maintain.
For us, the sweet spot is 100 to 200 gallons.