Separating zippers are easily shortened FROM THE TOP for your project. Just order top stops & apply to the last tooth before your neck or collar seam then cut off the extra teeth before tucking the zipper into your seam.
Molded tooth zippers come in #3, #5, #10 on my site.
#3 or 3mm is the smallest. Use in baby items or where you don't want a lot of bulk like a sweater.
#5 or 5mm is what you find in a light weight jacket.
#10 or 10mm is quite heavy & used for tents, screen porches, sail covers.
Metal toothed zippers are #5 & #10.
#5 or 5mm metal works well in fine leather jackets, hoodies or chaps.
#10 or 10mm is heavy for Harley jackets, coveralls.
Nylon coil zippers are #3, #4.5, #5, #8.
#3 or 3mm is small, light weight for apparel, evening wear. Reversed pull works well for invisible applications.
#4.5 or 4.5mm is a great weight for sweaters or light jackets.
#5 or 5mm is a standard for most jackets.
#8 or 8mm for bed skirts, bedding for your Airstream, sleeping bags, tents.