To answer this question we need to understand what each aspect references to. Tank size is the size of the specific tank that the individual organisms resides. Tank size usually refers to the size of the tank that the live stocks can live comfortably in (including swimming space). Total system refers to the total amount of water volume combined in the ecosystem. For example of you have three 50gal joined together, each tank is a tank size of 50gal, however the total water volume would be 150gal (assuming all tanks are completely filled and without rocks or sand).
Now that that's out of the way, there's no rule to what tank size constitutes a reef tank. The rule of thumb, however, is that the more the water volume, the more stable the tank tends to be. However when things go south, the harder it is to fix. There are reef tanks that run on 10, 5 and even 3gallons tank. Look up Pico and nano tanks. As long as you can keep up with regular maintenance and waste export, any tank size would work for a reef tank.
The next thing to keep in mind is what livestock you would like to keep in the tank. Every live stock has its specific tank size requirement for both swimming space and waste management. For example, it is unfair to put a tang in a Pico or nano tank. Some corals grow extremely large and might require constant pruning. Clams,on the other hand (like some other corals) can't be pruned and will grow very large.
To sum this up, figure out what livestock you'll like to keep, and have a tank that meets their needs. Hope this helps.
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