you could just cut a 11" in half,ending up with two pieces 11" wide and 6" long.most people think you have to cut in 1 ft. pieces,that serrated line is just a guide,you can cut width wise anywhere you want.
to the best of my knowledge flexwatt comes 3",4",11" 17" and 22" widths.
as far as what would work best that depends on type of rack,room temps etc.if in doubt it's best to be a bit over powered rather than under.
in my blanket box enclosed melamine racks with a room temp of 75-78 i use 3" 6 watt flexwatt and it works like a charm,BTW i would not use the 3" 10 watt as unregulated it can get upwards of 150 degrees.hope this helps,have fun!
edit:ALWAYS insulate the cut end of flexwatt.