Answers

What if the listing has no dimensions?

Type one number and you are fine. Width alone is enough to set the scale, and most shops publish it even when the rest of the spec is missing.

When a link is pasted, width, depth and height are pulled from the listing automatically. Some shops publish none of them, especially small independent sellers and second-hand marketplaces. That is the case this question is about.

Width is the number to enter. It does more work than the other two because it sets the horizontal scale, and everything else in the picture follows from it. Height helps for tall items like a bookcase or a floor lamp, where the proportion is vertical.

If you have nothing at all, the result is still worth looking at for style and colour, but stop treating it as a size check. Without a single real number, the placement is a plausible drawing, not a measurement of anything.

A quick way to find the missing number: the same product usually exists on a larger retailer with a full spec sheet, and manufacturers publish dimensions even when resellers do not.