Depends on what the load rating for the floor is really. Most tanks 40G or under are safe. Look at the bath tub, its usually the heaviest object on the floor when filled with water and most wont displace over 40 gallons with an adult inside, You could go larger but over time the additional weight will strain the timber beneath the floor and cause structural problems. Especially if you spill water on the floor and it sinks into the subfloor and timbers beneath, then warping can occur even faster. And if the structure becomes noticeably damaged you could be liable. Just something to think about.
There are some really nice small tanks too. Especially some of the nano designs.