A bag photographed alone could be any size, and the names makers give them — mini, medium, tote — mean nothing across brands.
The measurements that decide it
Length, height and depth, in centimetres, measured at the widest points. Strap drop — from the top of the shoulder strap to the top of the bag. This is what decides whether it sits under your arm or at your hip. Handle drop, for anything carried in the hand. Opening width, if you need to get things in and out quickly.
The practical question behind the numbers
Ask what it actually holds: "Does a 13-inch laptop fit?" or "Will an A4 folder go in flat?" A photograph with something ordinary beside the bag answers it faster than any dimension.
Belts
Belts are the clearest case of a label lying. The number on a belt is often the waist it suits rather than the belt's own length.
Measure from the buckle pin to the middle hole, and compare that to a belt you already wear. That is the only comparison that works.
Straps and chains
Ask whether a strap is adjustable and what the range is, and whether it is removable. A fixed strap at the wrong drop is a bag you will not use.
What to ask the seller
"Can you measure it with a tape in frame — length, height, depth and strap drop?" Questions to ask.
What protection covers
Dimensions materially wrong or a different bag than shown: seven days from delivery. A bag that turned out smaller than you pictured, when the measurements were given, is not.
Questions people ask
Are brand size names reliable?
Within one brand, roughly. Across brands, not at all.
Does a bag stretch?
Soft leather relaxes a little with use. Structured bags do not.
How do I judge capacity?
Ask for something ordinary photographed beside it, or inside it.
More: buying bags, describing condition.