knowing what size to get is the least of your worries.
Do you have an unlimited supply of free wood.?
Do you have a lot of spare time to cut and stack wood?
Do you have deep enough pockets?
Do you have neighbours?
Do you have a heating contractor will to do the install?
Do you have some way to haul and unload this beast?
Do you plan to include the installation cost, not just the unit?
Do you love to spend all your spare time cutting wood?
Do you have any other expensive hobbies?
Don't ask how I know these things. LOL