I know the meaning of one size fits all, but this is the first time I've come across the expression one size bigger. By using Google I know that you could say one size smaller.
But I don't know exactly the function of one in "one size". For example, can I say:"Do you have these shoes two/three size bigger?"
I think I don't have to make size plural if two/three is added, don't I?
Best Answer
The "one" in "one size bigger" refers to the number of sizes bigger, in this case one. If you're holding a pair of size 11 shoes and you ask for the shoes "one size bigger", I would bring you a pair of size 12 shoes (11 + 1 = 12).
You can certainly say "two/three sizes bigger", in which cases "sizes" should be plural, since you're counting the number of sizes.
Since we're talking shoes, you can also say, "half a size bigger".