Collectible figures are sold by scale, and a scale is a ratio rather than a size. Two pieces at the same scale can be very different heights, because the characters are.
Scale
1/6, 1/12, 1/144 and the rest describe the ratio to the real thing. Useful for knowing whether pieces will look right together; useless for knowing whether one fits your shelf.
Ask for the actual height in centimetres, with a tape or something ordinary in frame.
The box
For anything you will keep boxed, the box dimensions matter more than the figure. Ask for all three, because display space is usually the real constraint — and because the box also decides the delivery cost. Delivering collectibles.
Display space
Measure your shelf, including the height to the shelf above. The commonest disappointment in this category is a piece that fits the width and not the height.
Model kits and parts
Scale again, plus the finished dimensions if the seller knows them. For loose parts, ask for the part measured rather than named — part names vary by maker.
What to ask the seller
"How tall is it, measured, and what are the box dimensions?" Both, every time. Questions to ask.
What protection covers
Dimensions materially wrong or a different item than shown: seven days from delivery. A piece that is accurately measured and larger than you pictured is not.
Questions people ask
Is 1/6 bigger than 1/12?
Yes — the smaller the second number, the larger the figure. It still does not tell you the height.
Does the stand count?
Ask for height with and without it.
Why does the box matter so much?
For boxed collectibles it is part of the value, part of the display and most of the parcel.
More: buying toys, how they are graded.