Extend the "inside diameter" jaws located on a pair of veneer calipers between the inside edges of the gripping surface on the wrench. The gripping surfaces are the parts of the wrench that grip the bolt head. The reading on the veneer caliper will tell you the size of bolt the wrench can loosen or tighten.
Place a precision ruler on the surface of a table, then place the open-end wrench over the precision ruler. Place the left side gripping surface against the left side or "zero side" of the ruler. Take the measurement at the right side gripping surface.
Place a drill bit between the gripping surfaces of the open-end wrench. Graduate sizes until you reach a side that is a firm fit but still slips between the gripping surfaces. Inspect the shank of the drill bit for the size of the bit. This tells you the size of the wrench.