I measure a couple of ways:

1) I've marked my net in inches as a ruler.
2) I know the distance between my thumb in pinky when my hand is fully outstretched to be 9 inches (this was standard measure back in the day call a "span".) I've measured this.
3) Add to the "span" inches measure by thumb: the top joint of the thumb is about an inch, and the full thumb about 2 (This where the expression "rule of thumb" comes from.)
4) My booted foot is about -- wait for it -- a foot long.
5) For larger fish, I lay the along my rod with the tail even with the butt, and mark where the nose is. After releasing the fish, I then measure the distance to the mark with the tape I carry on a zinger.

(I'll sometimes use more than one of those methods, just to quality control my measurement. They usually agree within a 1/2 inch.)