For those times when TIG isn’t handy to write you a script, you can measure your geometry with native tools. For starters, as @DaveR suggested, turn on hidden geometry so you can see what you’re measuring. Sometimes I find it helpful to use temporary geometry to help analyze complex geometry. In this picture, I was easily able to measure the max OD of the ring by passing a couple of grouped (so they don’t stick) faces through the ring geometry. I just snapped the faces to the appropriate edges on center:

Unfortunately, in the process I discovered that your ring is constructed off-axis:

That’s going to be a problem from now on. You must rely on inferencing to keep things on axis even when you’re constructing something like a circle. Sorry, but I suggest you construct it over, on-axis. And this time, start with an edge or guide line as an axis so you’ll have that reference when you need it…

Since circles in SU are actually polygons, remember that the points (as opposed to the flats) represent the true dimension of the circle. In other words, when you measure a diameter, measure from point-to-point.

-Gully