6.5mm-7mm - very short names on 29mm+ blanks, monograms, use as first letter for name
4mm-5mm - names on 22mm+ blanks (longer names/words may need smaller or narrower font), word emphasis
3mm-4mm - names on tags <22mm, phrases
1.5mm - 2.5mm - longer phrases, items that are 3-4mm wide (cuffs, rings, necklaces)