Turk is right. The proper tool for what you are doing is a mic. And the mic set he mentioned is a bargain! (A single Starrett is probably more money)
But, you can get within a few thou with a caliper, enough to see if you have standard pistons or .030s or .060s.
You can also get a good idea about your rod dimension with your caliper.
But this thread has helped you learn where to measure stuff, and how to calculate what you need.
In other words.... measure a piston on the skirt (I usually measure center of pin high, with a mic) and piston deck height is top of piston to top of pin hole, PLUS half the diameter of the pin.
Next step is calculator, to see how it all might go together, what crank you need, and what needs to be done to it.
By the way, where's Buckeye? I'm in tri-state AZ.