Identifying fish size on sonar is very difficult since you can change the size with sensitivity adjustments or depth range. Fish look larger on a depth range of 20 foot than 100 foot. The size of a fish is determined by the color of the fish arch. For example if you have palette colors yellow, blue and red. The more yellow you see the bigger the fish. The thickness of the fish arch also determines fish size. How long the fish arch is just how long the fish is below the transducer.
In the image the black vertical lines show how long the fish is under the transducer and the green lines show the thickness of the fish arch.
The best way to tell size is catch one the fish you see on sonar or use an Aqua-Vu camera.
The first example shows my bait dropping (green arrow), the bait is intercepted by a bass (red arrow) and the bass swims to the bottom (black arrow) and then I set the hook and catch him.
This is the bass.
How about these fish. Needless to say I was excited when I found them but they didn’t bite so I dropped the Aqua-Vu camera and was surprised what I found.
On this screenshot is a 7.5 lb. northern pike, you can see when I set the hook and started bringing him to the surface.
The last screenshot is smallmouth bass and I caught a few, this is a picture of one of them. One of my favorite fish to catch.
This screenshot shows a 29.5 inch walleye after being released.
I was lucky in my timing to collect good data to prove fish size on sonar.
Last but not least is what a 5 inch minnow attached to a sinker looks like with 1.5-2.5 lb. walleyes below it.