Perhaps this should have been posted to comp.emacs.xemacs, but this newsgroup seems almost down.
My problem arose when my old laptop (windows XP, 1920x1200 screen) died, and I bought a new laptop (windows 10, 3840x2160).
I fell in the "high dpi hell", with the toolbar icons nearly invisible on the screen.
The edited text itself was made visible by choosing a good font size, but was blurred by the stretching mechanism of windows 10.
I decided the only way to go was to buy a large monitor (32", 3840x2160), in order to bypass the streching mechanism of windows 100, and display things at 100%.
Now the toolbar is correct, the text is crisp, but the menubar font is much too large, taking much screen estate. Moreover, as I added many items to the menu, it no longer fits in one line, making things unsightly.
I couldn't find in xemacs doc any way to change the menubar font. It seems the menubar font (size) is hard coded in C, and no lisp function allows to access it.
Browsing the Web, I found solutions for Xemacs under Linux, or possibly Cygwin, using the file .Xresources. I found also a solution for GNU emacs under Windows by setting register values under the keys
Sadly this doesn't work for Xemacs, searching the register via regedit didn't give me any key about menubar for Xemacs.
Perhaps I could create those keys and it would work, but I have no idea of the exact spelling of those keys for Xemacs !
I considered changing for GNU emacs, or Xemacs under Cygwin, but MY Xemacs is highly customized, including modifications in the elisp code for predefined functions, and it would be a very hard job figuring how I could reimplement all my modifications (more than 20 years of various adjustments) in a new software.
The best way for me is to set the menubar font size for Xemacs under native windows, as it is the only thing that doesn't work well with my new environment. If anybody out there has a solution, or some hints towards the solution, please help me.
Thanks in advance.