#1 never ask a salesman for advice on their product
hottubs are always on, a good cover should prevent some of the humidity but every time you remove the cover to get in the tub you're letting out some serious moisture
hottubs are said to be much cheaper these days than they were in the past, to the point that you don't need oil money to keep it warm 24/7, especially with a good cover...
what do you want the hottub for?
if you just want something to relax you can get one that fits in roughly the size of a standard tub... the more accessories/jets etc, the larger they get
if you want something you can enjoy with a "partner" i wouldn't enev think about getting one to go in your bathroom, they might be bigger than a normal tub but no where enough room to do your thing...