These two have been resolved, and I've made a new Windows (msys2) snapshot build of freeciv-2.6 development.
See thread about those: viewtopic.php?p=107694#p107694
This initial support to use font size from the theme is very crude, but I just tested that it is functional in this build. Text file data\themes\gui-sdl2\human\human\theme\theme.themspec under Freeciv sdl2-client installation directory has line "default_font_size = 10" which one can adjus. Next time you launch the client, it will use that font size for many things, but not everything (basically, anything that had size different from 10 to begin with, is not controlled by this)
Also, Windows makes this a bit hard:
1) On default installation location administrator rights are required to modify/rename/copy files
2) File must be text only, no formatting of any kind (notepad and wordpad should be able to do the job)
3) Windows tries very hard to add suffix ".txt", and does not even show that it had been added