A fresh donor tissue valve isolation annulus size comprising a handle and trigger or crank knob manipulated by the user moves a movable probe into proximity with a stationary probe so that the probes can be inserted into the tissue valve lumen. The user releases the trigger or crank, and a biasing mechanism urges the movable probe away from the stationary probe until the resistance presented by the valve annulus equals the biasing force. A gage coupled to the movable probe displays a measure of the valve annulus diameter as a function of the distance that the movable probe moves away from the stationary probe until restrained from further movement by the fresh tissue valve annulus.