That length of snake, I'd say 28 quart with two hides (warm side and cool side) and you should be fine. If he/she is nervous, you can always black out the sides of the tub with posterboard cut to fit and then taped over it to give it a more secure feeling. As to UTH size...4 quarts in a gallon and with a 28 quart tub, that'd be 7 gallons?, so if anything, a 10-20 gallon size UTH heater would be more than you'd need but that way you wouldn't be maxing out a smaller unit trying to heat the tub. Whatever you do, ABSOLUTELY get a thermostat to regulate the heat output/temperature of the UTH.