There must be a fairly easy match problem to solve object size under the boat as you pass over.  How and what is it?

 

Boat speed 2.5 mph

Depth 22 feet

7 inch screen

Object takes up 25% of screen on the bottom

How wide is the object?  I want to make a speed and depth chart for easy use.

My Humminbird has a ruler tool that can be used to measure distances.  I don't know if your Garmin has something like that. 

 

There are A LOT of variables that go into the calculation you're wanting to do.  The chart speed which should be adjustable on your unit must be part of the calculation.  The shape and width of the sonar beam is also a factor.  Are you driving directly over the object or is it to the side and how far? This will matter if you're using 2D cone shaped sonar.  It's not simple math.  In 2D sonar,  a one inch fish can appear on the screen as a 2 foot arch because it was in the sonar cone while the boat travelled 2 feet.

 

You can get the coordinates of where you started seeing something on the graph and again when you stop seeing it and then calculate the distance between the two.   This will give you a general idea of the size of large objects to within about 1 meter assuming you're using down imaging sonar.  There's more complicated math to get higher resolution measurements that I sometimes do at home when viewing recorded sonar files. 

 

When I'm on the water I usually just guess at the size and then do the math when I get home just for fun. ?