Your bore oversize can be determined by using  dial, vernier or digital calipers.  They cannot be used to measure the condition of the bore because they will not reach the areas that experience the majority of the ware.  These areas of high ware can be reached with a bore gauge, an inside micrometer, or telescoping gauge.

Calipers are generally used for quick measurements not requiring accuracies less than a couple of thou.  Because some calipers have .0005" increments, does not mean they are actually giving you an absolute dimension that could be traced to a standard