Zipper size can be recognised according to the number embossed on the slider back. However, at times these numbers may indicate the moulds they are cast from. In this case, you would have to measure the zipper size in person.
Zipper size refers to the teeth width when it is closed. Size of the zippers is stated in #, and depending upon the measurement systems in different countries, different units like millimetre or inch may be adopted for measuring the width of the zipper teeth.
Zipper size can be measured easily by following the steps below.
1 Place the zipper on a horizontal surface.
2 Fully zip up the zipper.
3 Use a tape/ruler to measure the width of the zipper teeth, inclusive of both sides.
The measurement would amount to such numbers as 3/4/5/7, etc., with a slight deviation. For instance, the teeth width for a 8# zipper will be about 7.2-8.0 in millimetres. If the number is larger, then the teeth width is wider.
Please refer to the image below for the different sizes of a plastic zipper.
If the teeth width is wider, then it would strengthen the cross-wise chain performance of the zipper. To put it simply, if the size of the zipper is larger, then it is sturdier. This would indeed allow thicker and heavier fabrics.
The specification charts below present the standard size and teeth width of metal, coil and plastic zippers respectively at SBS Zipper Company