A label size tells you what a maker called the garment. It does not tell you what the garment measures, and those two have been drifting apart for decades.
Why the stated size is not enough
Makers differ, and the same letter means different things in different countries and different decades. Alterations change the garment without changing the label — a taken-in shirt is a smaller shirt with the same tag. Shrinkage does the same thing more slowly.
The measurements that decide fit
Taken flat, with the garment laid out and not stretched:
Shirts and jackets — chest armpit to armpit (doubled), shoulder seam to shoulder seam, sleeve from shoulder seam to cuff, back length from the base of the collar. Trousers — waist flat (doubled), inseam, rise, leg opening. Knitwear — chest, length, sleeve.
The habit that solves this permanently
Measure one shirt and one pair of trousers that already fit you the way you like, flat, and keep the numbers on your phone. Every future decision becomes arithmetic instead of hope.
Two centimetres across the chest is noticeable. Five is a different garment.
What to ask the seller
"Can you measure the chest flat with the tape on camera?" — and whatever else matters for the garment. Also: "Has it been altered?" Questions to ask.
What protection covers
Measurements materially wrong, or an alteration not mentioned: seven days from delivery. A garment that fits differently than you hoped, when the measurements were right, is not.
Questions people ask
Should I ask for both the size and the measurements?
Yes. The label helps people search; the measurements are what you buy on.
What if the seller will not measure?
Bid as though you have no fit information, or do not bid.
Does fabric change the fit?
Yes — a rigid cotton and a soft linen at the same measurements wear differently. Ask what the composition label says.
More: buying menswear, describing condition.