New user in need of pool heater recommendation

Hi everyone, this is my first post. I know some about pools but not a lot so here goes. My old heater is leaking. It looks like the heat exchanger has rotted out so I'm told better to just replace. The unit is original so that makes it about 16 to 17 years old. It heats a 15,200 gallon pool and spa. We live in Dallas so we never use it to heat the larger poo just the spa. Now for my question. Can I get a smaller unit that will be dedicated to just the spa part of the pool and leave it at that? Thanks for your thoughts.