Japanese For Busy People volume II, revised third edition, on page 23 has
To state or ask how much bigger, you use other words.
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".