valley11,
Depends on how fast you want to heat the water. 100K to 400k will work. Check the heater charts for a difinitive answer based on your gallonage, flow rate, air temp. Info is available on most heater websites; Pentair, Sta-Rite, Hayward, Jandy, etc.
Be prepared to spend a couple grand+ to get it done.

Thank you. I think we have narrowed it down to between Pentair and Raypak. Any ideas on which is a better product?

Consider solar mats for heating as well if you haven't already. We are in the SE area of Wisconsin. Have a 27 foot - 54 inch above ground and use 4 2 x 20 mats. Last year we averaged 84 - 86 without the use of the solar cover. We are pretty windy here and should use the cover but hate pulling it off and putting it on.

valley11,
Both Pentair and Raypak or good product choices.

Thank you all for replying. We are thinking we may do a heat pump instead. We aren't sure which to do. Any opinions?