Not sure exactly how your calipers work or if they're calibrated but if you read the the zero 0 line on the mm side, it reads 86mm.

Now 86mm converts to 3.39 inches (86/25.4mm = 3.39")

Also if the piston is a Weiseco the diameter should be laser stamped right near the center of the crown.

You can order a set of 9 inch digital calipers from Harbor Freight for $10 on sale and they switch back and forth with the push of a button.  They'll will save a you a lot of headache.