"I don't have a bigger one; it's one size fits all."
Translation:Je n'ai pas plus grand, c'est une taille unique.
I don't think that would be a correct translation. (It's missing the part of the sentence after the semicolon, but I assume you just aren't asking about that bit.)
What you have reads to me as "I don't have the one that is larger," i.e. I don't have that one specific item that is larger, as opposed to the required "I don't have one that is larger", i.e. any item that is larger.
Also, if I give Google Translate "I don't have the one that is larger" it gives back "Je n'ai pas celui qui est le plus grand," so I believe the lack of definite article in your sentence is a grammatical error.