With GIMP, it is possible to select and copy a region of the image to the clipboard, and then use this region as the brush for the Airbrush tool.
However, the size of the 'painted' region, is controlled by the Airbrush size, and will scale (I believe) the region in the clipboard to match this.
Other than setting the size manually by eye, is there a way to make the Airbrush 'spray' a region with a size identical to the original?