I don't use this feature because I like Godot's script editor.
I only used it once, because I wanted to have both my 2D scene and the code displayed at the same time. Then Godot was on my left screen and Notepad++ on the right screen. However, Notepad++ doesn't have any color module nor auto-completion adapted to Godot, it is definitely not the best one to use.
I know Atom has some modules for Godot, but I never used it.
I didn't find any way to change the font size, but maybe someone will! That's why I didn't answer, I just added a comment instead.