It is in CFS1/FS98 and CFS2/FS2k; I'm sure it is in FS2k2, as well -- probably in the others (don't know your version).
This can be done a couple ways, depending upon the panel format. If you don't wish to modify (skew command) a copy of your bitmap and (note: choose not to enlarge the old bitmap, itself) paste and position it over a another copy of the old, there should be a screen-sizing entry at the bottom of the panel.cfg. However, this means losing a part of the original proportional to the amount of enlargement and viewed screen size.