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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".
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2018-03-06T09:39:25.210
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question
meaning words loanwords
Is ワンサイズ a valid Japanese word?
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](https://eow.alc.co.jp/search?q=%E3%83%AF%E3%83%B3%E3%82%B5%E3%82%A4%E3%82%BA).
Although uncommon, you can say ツーサイズ, too. A non-katakana equivalent would be
一段階.
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answer