Check your Owners Manual but I would bet your tractor come with 15 inch front tires. If you don't have a manual post the 917.????? number under the seat.
Cannon
Link to manual at Sears Parts. Manual Craftsman Tractor 917275810 | SearsPartsDirect
Cannon
If they are only dry rotted, take a look at the inner tubes. I got one for a rear wheel for 13 and installed for 15.
Cheaper than new tires.