First you have to find out what width tire will fit in your bike. The swingarm width is a pretty hard limit, the original rear fender (particularly the mounting bolt area) is probably a bit thinner, and the shocks may be the tightest area. Once you hit the chain you're into a lot of tricky work to get a bigger tire on the rear, so that and the swingarm will be the serious limits. The street tracker pictured is running solid rear struts, not unusual on some dirt track bikes but odd for a street tracker so possibly he had to take the shocks off to clear the tire. The front fender and stays cause the most friction when trying to install a large front tire, the front end can get pretty loose once the fork brace action of the front fender bracket is removed though. Getting a fat tire past the disk caliper(s) can be a hassle too.
Once you find the tire width you want, you check out tires and sizes to get approximately the outer diameter you want. Again there are limits to look at, you don't want the front tire to hit the triple tree at full fork compression in particular.
For an example, say you can fit a 180mm tire width and you want a 26" outside diameter. With a 70% aspect ratio tire, this gives you about 5 inches of tire off the rim, so you'll want a 16" rim and a 180/70-16 tire.
Once you find the tire that you like, find the rim width that's recommended for that tire. Then buy the rim and spokes you need and lace it up.
Buchanans has a good selection of steel and alloy rims with the required spoke drilling and can get you the spoke sizes required for non-stock rims.
