My tire wall tells me that my tires are 26 x 1.95. I bought a 26 x 1.75-2.15 inner tube. When I change the inner tube, though, the tube is bigger than the wheel by two inches. How do I determine inner tube size? Or am I just doing this wrong?
That tube should be the right size, based on the numbers.
If you inflate the inner tube outside of a tire it will generally balloon up quite a bit, becoming quite a bit bigger than the tire.
When installing a tire and tube you should generally just inflate the tube to the point that it "fills out" -- not really much more pressure than you could blow into it with your mouth. Then the tube should fit comfortably inside the tire.
If a tube is too big in overall diameter there is a danger that it will "telescope" inside the tire (similar to "intussusception" of your intestines), resulting in a lump and, eventually, a split in the tube. Having the tube inflated too much while installing it is one way you can get this situation. (The other is simply having a tube that's too large.)