Seiwa & Kyoshin ElleThese pricking irons are measured as the distance between the points i.e. The stitch length.
They don't use a width for the iron, they use a number of prongs (1, 2, 3, 4 or 6).
These are expressed as a x b mm on the store, so 3.5mm means 3 prongs with 5 mm stitches etc.
The stitches per inch = 25.4 / stitch length:
3mm ~= 8.5 stitches per inch
4mm ~= 6.3 stitches per inch
5mm ~= 5 stitches per inch
6mm ~= 4.2 stitches per inch
Craft ShaThe Craft Sha irons are measured as the distance between the prongs. these are half the distance of the ones above, so
1.5mm ~= 8.5 stitches per inch
2mm ~= 6.3 stitches per inch
2.5mm ~= 5 stitches per inch
3mm ~= 4.2 stitches per inch
Again with the Craft Sha the first number is the number of prongs, so 10x1.5mm means 10 prongs giving 8.5 stitches per inch.
These numbers are not my own, ALL Credit goes to Ian Atkinson for doing the leg work, I have simply transcribed the work he's done here.