Trouble matching a screw - are there sizes I may be overlooking?
If your located in the States or Canada most light fixture screws will be SAE or imperial measurements. The most common screw size used for fixtures will be a 8/32. The next sizes closest to this are the smaller 6/32 and the larger 10/24.
Why not shorten the original screw? Thread the nut down the screw to below the length yyou want it to be. Cut the shaft to the desired length. Unthread the nut so it straightens the cut hreads. Voila!