Algodoo

Could you make something that you specify the length or the input crank, the coupler the rocker and the bottom thing and it creates a polygon that is the locus or a mathematical equation for that?

Yes, I could make something that does that. I'm not sure if I will. I assume the locus is of the coupler center point. Creating a polygon would be easier than deriving the mathematical equation.

A polygon making tracer point might be an interesting project.

Here's how to create a polygon form a coupler curve manually:
1. Select the tracer from the coupler curve of interest.
2. Add the following code to the tracer:
_ra := [];
update = (e)=>{
sim.running ? {
_ra = _ra ++ [pos]
} : {}
};
3. Start the scene.
4. Stop the scene when the tracer gets back to its start point.
5. Create any polygon.
6. Copy the polygon from Algodoo and paste it into a text editor (Ctrl-C, Ctrl-V).
7. Copy _ra from the tracer (Ctrl-A, Ctrl-C).
8. In the text editor, replace surfaces := [...] with surfaces := [_ra(Ctrl-V)].
9. Copy everything in the editor (Ctrl-A, Ctrl-C).
10. Paste into Algodoo (Ctrl-V).