In Photoshop you can directly specify the brush size like you can specfify directly the "spacing" of a brush in the GIMP. I would really like a slider/box when selecting (clicking on) a brush like there's one for spacing, but FOR BRUSH SIZE. 
Here's an example from Photoshop: http://www.eyesondesign.net/pshop/pattern_wire/13px_brush.gif

As you can see, you can directly select the "diameter" with the slid-bar or with the box (both should be). I'ts really TIME CONSUMING having always to create a new brush, save it, and then only use it, and if you want just to make one line, it's really not worth all this. Also, with the time the amount of different brushes may grow and grow... and everything becomes messy. Still, presets are useful, but only few.
So the solution would be to make it like in Photoshop, selecting a brush TYPE (preset, etc...), and then selecting with the slide-bar/box the size of the brush without having to save it so I could use it DIRECTLY; WITHOUT LOSING TIME :-)

Thanks