More custom kit orders go wrong on sizing than on print, fabric, or delivery combined. The garments arrive exactly as specified, and half the squad still cannot wear them. Almost always, the cause is a size chart nobody actually measured against.
Manufacturer sizing is not retail sizing
There is no international standard for a size Large. A Large basketball jersey cut for the North American market may be four centimetres wider in the chest than a Large cut for a European club, and both are correct for their audience.
This means the only size chart that matters is the one belonging to the factory making your order. Never assume your new supplier grades the same way your previous one did, even if the numbers on the label look identical.
Measure the garment, not the athlete
Professional size charts specify garment measurements laid flat: chest width from armpit to armpit, body length from high point of shoulder, sleeve length from shoulder seam. Body measurements are a starting point, but garment measurements are what you can verify when the boxes arrive.
The practical approach is simple. Take a garment your squad already wears and likes the fit of, lay it flat, measure it, and send those numbers to your manufacturer. Any competent factory can work backwards from that.
Order a size set before bulk
A size set is a single garment in each size, produced before your bulk run. Your squad tries them on, you confirm the grading works for real bodies, and only then does production start.
It adds roughly a week to the timeline and saves entire orders. For a first order with a new supplier, or any order above a couple of hundred units, it is the most valuable week you will spend.
Plan the size spread properly
Collect actual sizes from every athlete rather than estimating. A common distribution for an adult mens squad runs roughly 10 percent Small, 25 percent Medium, 30 percent Large, 25 percent XL, and 10 percent 2XL, but your squad is not average — a rowing team and a soccer team look nothing alike.
Build in a small buffer of the two most common sizes for mid-season additions and replacements. Reordering three garments later costs far more per unit than including them now.
Do not forget fit preference
Compression, athletic, and relaxed cuts in the same nominal size fit very differently. Agree the intended fit with your squad before design, and state it explicitly on the order. It is the cheapest correction you will ever make.