a few years ago,
I am pretty sure there was a pic circulating
of a J-20 (maybe a mock up) at an air show display or something
that had a sign with its official dimensions.
You sure not mistaking it with J-31 ?
it was definitely the J-20 because the pic was used in context with that debate over the J-20's length.
its somewhere on keypublishing
I don't recall any official display of J-20 at an airshow that describes its length.
The only official brochure of J-20 we've had afaik is this one, from Zhuhai airshow 2018 but it doesn't describe the aircraft's dimensions, only its general role
The other thing you might be thinking of is this non-official render and some specs about a hypothetical carrier based J-20, but this is not official and a fan made piece of art.
Apart from these two I'm not sure what possible brochure you might be talking about. Fwiw, if it exists, I can't imagine the rest of the PLA watching community would've missed it.