Ok well assuming running at the most two elements at once would be 11,000 watts then. The 60A would allow enough leeway to run the pumps and other small loads as well. 50A would be cutting it close. Goingr to 60A gives the extra 20% allowance for continuous loads (more than 3 hours) and depending on how long your run is getting #6 AWG Cu isn't going to cost all that much more than #8 so I'd just go #6 now.
Of course you could always go even larger over to a subpanel or local disconnect or spa panel, if you thought you'd want to run all three elements that'd be 16,500 watts, so about 69 amperes. Allowing the extra 20% would give about 83 amps so at that point you're looking at #4 or to go up to 100 amps #2 copper, that is going to start getting much costlier per foot. Plus that might cause you to want to bump up your service to the house, like if you were getting a 100 amp service you might want a 200 amp service instead.
