Damiem, You have to do reverse engineering. Do you want a skinny 21"? Do what Biker said and look at the tire manufacturers fitment charts a see what rim width they suggest. As far as the spoke length, if you get the rim, place it on top of your existing wheel, line-up the spoke holes with some of your spokes and see if you can measure from your hub to the new rim. Who knows, Buchanan probably can help you or even know off the top of their heads the length of spokes you need!! I just did a couple of peddle bike rims in my youth.
Enjoy the ride,stop and smell the flowers and ride safe
