@Natan - Thanks for this
I have bought about 400 plugins over the last years.
I have made many screenshots and looked what size the individual plug-ins have.
Since a few years the GUIs are getting bigger and bigger - which is of course due to the bigger screens and resolutions. Also, most modern VSTs can be scaled the way you like it.
My decision was 1200x700. On the PC no problem, On my old MacPro (2013)
with only a single background image is hardly operable. - Where only in the backend -the compiled PlugIn works with OpenGL turned on.
The user can scale the window so that people with laptops can simply scale to 75% or 50%.
the 4K user to 150 or 200%.
That's what scalability is for.
That this doesn't work on my old Mac is really due to the graphics card and the limited OpenGL support.
As already mentioned, I will solve the problem by programming on the PC and editing the graphics on the Mac. Since I have a server on which all data is stored, this works quite well.