Five or ten gallons if "monster" and "passion" are synonymous terms. By some peoples' definition, if you are passionate about fishkeeping, then you are a monster fishkeeper so therefore, all of your tanks would be monster tanks. Perhaps the word "monster" should be defined for this setting. It could be an adjective for something extremely large, or it could refer to something with defective or excessive growth--such as giants or dwarfs. Defining what a monster fish is would make it much easier to come up with a minimum tank size. But since every one on this forum is a monster fishkeeper, all of our tanks are monster tanks. 35 gallon tanks are monster tanks.
If we are talking about a tank to house large fish, that is still open to interpretation. Some fish are bigger than others. Our 300 gallon tank is too small for our two pacu, but the tank would be fine for smaller fish. If we are talking about extremely large tanks, perhaps it would be better to call them "colossal" tanks. To me, a colossal tank would be 4000+ gallons. I will never have a colossal tank, but then, again, I don't need one. Fish tanks are like cars and houses--no matter how big yours is, there will always be someone with a bigger better whatever.