The most common cause for lettering issues in the editor (too small or too large) is due to the user zoom setting if that is combined with other scaling issues it may be producing odd effects.
So the first thing to try is use CTRL and the scroll wheel to adjust your zoom to a reasonable level.
The most likely cause when using dual screens for a mismatch in dpi settings is
the system trying to auto match for the differences Preferably both screens should be set to 100%. In this case there are a few advanced editor settings so try this combination.
Finally if you messed about with other font settings you may need to reset
so try this first beware it may remove some of your custom settings so it is a good idea to save profile first that is two lines above on the menu (you can then reload if necessary later)