@deutrino Aaugh, that makes it sound so complicated. Not actually that hard.

Steps:
1. Choose the ellipse tool
2. Drag
3. While dragging, press and hold the shift key
4. While holding the shift key, end the drag at the desired size
5. Either choose Edit > Stroke Selection (for unfilled circle) or Edit > Fill with FB (or BG) color (or do both, if you want both)

#gimp