Is ワンサイズ a valid Japanese word?

Question

Japanese For Busy People volume II, revised third edition, on page 23 has

To state or ask how much bigger, you use other words.
ワンサイズ大きいのはありませんか。
Do you have one that is a size bigger?

However, the word (as of today) doesn't exist on forvo.com, on jisho.org (either as a word or in its corpora), the English or Japanese editions of Wiktionary, and on weblio it's only mentioned in sentences.

Is ワンサイズ a valid Japanese word, in that it's easily understood, unambiguous, and the most common way of expressing the concept of a difference of one size?

The only explanation I can think of as to why it's valid but not in dictionaries is that it's merely regarded as a combination of the words "ワン" and "サイズ".

Answer 1

Yes it's perfectly valid as a Japanese expression and is understood by virtually everyone (except for very old people). ALC has several examples of ワンサイズ. Although uncommon, you can say ツーサイズ, too. A non-katakana equivalent would be 一段階.