Several of the larger Swiss Army knives from Victorinox include a combination fish scaler, ruler and hook disgorger.
The tool is about 3" (74mm) long. It's quite effective for removing scales from panfish, trout, bass, catfish, and about anything else you'll catch during casual fishing expeditions. If you are a serious angler scaling large quantities of fish, you'll probably want a larger tool!
This tool also includes a hook remover on its tip, and a ruler on the body (inches on one side, metric on the other).
Rub the serrated part of the blade back-to-front on the fish to remove scales. You can also use the prong at the end of this tool to grip fish hooks and pull them out.
The ruler is a bit small for measuring real fish directly, unless you are a minnow fisherman! However, just follow these steps to measure longer things: