Steel wheels

As far as I know, all small bolt pattern wheels take the small 8 3/4 / 9" caps.
As for wheel width with tires on, I will try to explain my method. (Note: rim width is measured between the beads, so an aluminum wheel 15x8 will usually be 9" overall).
With the tire inflated, lay it on the ground, back side up. Put a straight edge across the tire, and measure to the rim lip. Say that measurement for simplicity sake, is an inch. It will be the same, front and back.
Now measure from straight edge to the ground. If you get 8", for example, that means the wheel is 6" flange to flange,(8-1-1) and probably a 5.5, if a steel wheel.
Edit Same technique can be used for backspace measurement.