A Caliper can be used to determine the size of a bolt or stud / shock mount. Or the size of a hole / loop at the end of a shock (the inside width / diameter).

If your caliper measures in thousands, just measure to the bolt's shank (NOT the head) and hold that up to your tape measure to convert to metric or English sizes.

The most common tool to use is the Tape Measure. This can be used to determine the extended and collapsed length of a shock absorber.

You can also use it to measure the hole / loop at the end of a shock, measure the "feet" width, or the length of a threaded stud mount.

As a short cut, if you want to determine the size of a bolt or stud used on a shock mount, you can put a Wrench to the shank (again, not the head) of the bolt / mount stud.

If a 9/16 wrench fits snug on the bolt, it's a 9/16 bolt. (metric wrenches of course can be used accordingly for metric bolts, etc.).