The font is determined by the .skin file, sadly that means quite a bit of work to change it, however it can be done if you are persistent. Download/install
reshacker 3.6 Then open your skins folder in the classic shell install directory. and right click the skin you are using and select open with reshacker. then search the code for the fonts and change them to the size you want. Save to the desktop (reshacker cant save in the skins folder) then copy the file from the desktop back to the skins folder) restart classic shell and it should be changed