Scott440,
Stumping depends on many factors: Type of ground ( clay, rocky, or sand ), Type of tree ( some root shallow and some go deep, some use side roots or tap root or side and tap roots ), machine horsepower and machine weight, and how much time you are willing to invest per stump.
For example. I cut a 20 inch, at base, hickory where I want to expand my driveway. Dug around stump trying to break roots with
M59 and 12" bucket with 7600 lbs bucket curl force. I broke a few side roots and then gave up after 2 hours of digging. Just recently purchased a root ripper to go back and attack that stump. Looks like this.
Most any tree up to 5 inches in diameter, I can pull out of the ground by grabbing trunk with backhoe thumb and pulling up with some back and forth motion.
My gut says 24 horsepower, with associated machine weight and curl power, isn't going to allow you to stump anything larger than 6 inch in diameter.
Have fun learning your machines limits.