I'm the customer service guy here at VO. Odds are if you call or email us, I'm the person who will answer your question, solve your problem, or suggest a web site to look at. Looking back at the questions that we are asked, one item that would help a lot of folks answer their own question or at least have a good starting point, is a set of digital calipers. All of us here have a set on their desks. I use mine multiple times over the course of a day to measure things for customers or just to get a perspective of how large or small something is.
Digital calipers are the modern day evolution of Vernier calipers. You can use these to measure the outside width of a bar or the length of an axle or the diameter of a seat post. They have smaller measuring jaws on them so you can measure the inside diameter of a bar.
The digital calipers can be converted at a push of a button from metric to inches as a decimal or as a fraction, thus allowing for easy conversion from one to another. There is also a set screw on the top so that you can hold a measurement if the reading is turned away from you.