The proper luff tape size is the one that allows your sail to slide easily up your headstay foil is snug but not too snug.
You can easily check this by using a (metric) drill bit and inserting it into the foil.
If you can just fit a 7mm drill bit into the foil extrusion then a luff tape that is 1mm to 1.5mm smaller should be selected. By selecting a luff tape slightly smaller than the maximum possible for your system you ensure that the luff of the sail can easily slide up the furling
The most popular luff tape sizes are # 5 and # 6.
Luff tape is sold in metric measurements this means that imperial measures will never match up perfectly.
➥ Furler Luff Tape