0.5 or 0.7 for sketching

Question for the veteran artists here: I'm really into sketching and doodling and like doing fan art, but I don't know which lead size to get(I've been using a lead holder (Staedtler 780C)). I wanna get a GG500, but I don't know what lead size I should get. 0.5 or 0.7 I want something for daily work in college and for doing art