If you open up your Effects folder in FS, you will see 3 effects for cockpit lighting; fx_vclight.fx, fx_vclighth.fx, and fx_vclightwhi.fx. Any of these could be edited to get the desired size/color you want, although they should of course be backed up first (I am not an expert at playing with effects). Before doing this, test each of these effects by renaming the appropriate entry in your aircraft.cfg lighting section - you may find one that does the trick.

which software are you using?If you are using fspanel studio there is settings in there for gauge lighting and changing gauge colours.
Hope this helps