If I set the scale to 100% on one toon, then switch to another the interface will say 100% is set, but the UI will still be larger.

UI should be 100%, but it's closer to 130% I think.

UI says 100%

Bump it to 140% to show issue..

Now back to 100%...

And then open trade window... and RM is the right size.

But then when you log in with another toon, it's back to large again.

Hello, @Gogo1951.

I appreciate the detailed step-by-step, but I wasn't able to reproduce the issue.

The scale of Recipe Master's main frame is determined by the effective scale of UIParent plus the value of UI Scale set in the options frame (which translates to between 0 and 0.4) in EventHandler.lua.

Unfortunately, I'm not able to accurately test this behavior since your resolution is way higher than my current monitor resolution.