Measure the length of your rods grip and the length from the end of the reel seat to the end of the grip. measure the end of your grip to the first guide or maybe to the hook holder. That should give you a pair of known distances, probably about 7 and 12 inches. You can use those numbers as a reference measure fish against your rod. Another useful length is the opening of your net and handle length. If you want to get a more accurate measurement try finding a pair of guides the right distance apart and measure the gap when you get home. Or you can just say it was nice fish and let it go at that.