There is no way to tell the required size of the boiler based on what you gave...while there are some WAGs based on square footage, they are usually way off. You need to know the exact amount of insulation, window location and sizes, orientation, and the needed design temp along with the heating degree day info for your specific area. You'll save money and grief if you pay someone to do this but, not everyone is capable of doing it, or willing to do it right (they'll just give you a WAG). You can figure it out yourself if you have fuel use history along with the HDD info, which you can download for your area. Each pound of propane provides a known (or pretty good estimate) amount of heat. That coupled with the efficiency of the existing boiler and you can see how much heat is currently being used to heat the place. That sized boiler could be as much as 3x bigger than needed, and if so, would be lousy in efficiency and longevity. The only way to tell is to run some numbers, and you've not given enough info to do it.