Axle nuts are pretty easy. They're usually numbered like this: first number is length in millimeters. Second number is diameter in mm. Third number describes no. of threads per inch. 24 threads per inch is pretty standard stuff. Most new rear axle bolts are 10mm diameter. You're going to have to get a measuring tool to make sure of what you got. A cheap Chinese caliper is about $10. Borrow one or, you can also just use a plain old ruler. A new rear axle can make a bike run like new. Be good. Have fun.