The config file has some settings that might help

https://github.com/0ad/0ad/blob/master/binaries/data/config/default.cfg

Such as

scale = 1.0                       ; GUI scaling factor, for improved compatibility with 4K displays

And maybe

; Enable Hi-DPI where supported, currently working only for testing.
hidpi = false

Finally

; Force a particular resolution. (If these are 0, the default is
; to keep the current desktop resolution in fullscreen mode or to
; use 1024x768 in windowed mode.)
xres = 0
yres = 0