From what I have read, 34qt tubs are okay for male adults and small female adults, and smaller sizes depending on how much security you are offering them.

3" flexwatt is the most commonly used size from my research. I just ordered 32ft of 10watt/ft 3" for the rack I am going to build. 3" also comes in 6watt/ft. 11" and 17" seem to be used as back heat primarily versus belly heat.

I would also suggest getting a proportional thermostat. You tell it what temperature to be at and it will adjust constantly to maintain that temperature. Dimmers are a pain in the butt and require constant attention and adjustments.