Hmm really? I'm from the UK and it sounds like a fairly normal thing to say if someone gave you the wrong shoe size (as an example).

What would you be more likely to say in the US?

If someone handed me a shoe that was the wrong size, I'd say "this" and not "that" for starters.

I'd more likely just say, "These shoes don't fit." or "These shoes are way too small." (... or way too large.)

Anyway, like I said, I don't know German.
I sort of suspected that maybe the German was an idiom of some sort and shouldn't be translated literally.

Das ist eine ganz andere Schuhnummer.

"Das ist eine ganz andere Schuhnummer" is indeed some kind of rarely used idiomatic expression, which I used in one of my recent comments and Cedric (sacredceltic) recycled it here, so to speak.

It's similar in meaning to "This is a different kettle of fish".

"shoe size" is normally rendered as "Schuhgröße" in German.

So where do we go from here? Either unlink the sentences or think of replacing "shoe size" by "kettle of fish" maybe.

Thanks for the explanation, it's always great to learn some more odd idioms :-). I've changed the translation to "This is a different kettle of fish". I hope that's alright?