You have been looking at old information, probably for Gimp 2.6 where there were lots of pre-made brushes of different sizes. Still there in Gimp 2.8 but hidden.
What you are using is the brush editor for creating Gimp parametric brushes. you can do that if you want to, save them in your Gimp profile brushes folder.
To change the size of any brush you now use the size slider in the tool options.
edit: There are other ways of changing the brush size. Try the square brackets ] [ which increase one-pix-at-a-time then withctrl or shift for large steps sorry not for Windows users.
What you are using is the brush editor for creating Gimp parametric brushes. you can do that if you want to, save them in your Gimp profile brushes folder.
To change the size of any brush you now use the size slider in the tool options.
edit: There are other ways of changing the brush size. Try the square brackets ] [ which increase one-pix-at-a-time then with