In the main menu Settings -> Game -> Game Interface Scale. It claims to default as follows:
local realWidth, realHeight = Spring.Orig.GetViewSizes()
local defaultUiScale = math.floor(math.max(1, realHeight/950))*100
local maxUiScale = math.max(2, realWidth/1000)*100
local minUiScale = math.min(0.5, realWidth/4000)*100
return defaultUiScale, maxUiScale, minUiScale
Essentially, every 950 screen height you go above 950 steps you along 100%, 200%, 300% etc..
My guess is that the default is still working, but when you change your screen the value isn't updated. I think this is ok.