Hi,
I haven't found any option that let us choose an arbitrary resolution for screen shots. Screen shots get automatically the windows size (I know, it is normal...). So taking low res screen shots don't cause troubles (At worst case we can reduce them with gimp).
But, is there any way to take hi res screen shot. Perhaps with recompositing many low resolution screen shots ? In openGL there is a way to get subpart of an image by dealing with the frustrum and viewport, but as I don't have the time to explore the celestia code. So if there is a "simple" solution...