Examples from my 1440p monitor. You results may vary.
Did you upgrade your monitor and now the font on your hotkeys is TOO SMALL? Just download and copy the file of size you want to try 2, 3 or 4 (default is 1) into your custom UI folder.
These are just the default files where the 1 in <Font>1</Font> was changed to 2, 3 or 4. You can do it yourself and if you need larger can do larger. If you need larger and need help /tell povar.strumph