The size of a sector is about as ambiguous as the size of the UFP... But 20 ly grids were favored in the DS9 background maps, FWIW. That is, by knowing that the distance between Bajor and Cardassia was supposed to be about five lightyears, we can see the artists used a 20 X 20 ly grid (or sometimes 20 X 40 ly) - but in canon terms, this is a circular argument because nothing in canon has established the exact distance between Bajor and Cardassia, either.
Back in ST2:TWoK, having two starships in the same sector was already puzzling... Although Sulu's surprise might have been more due to the Reliant
not announcing her presence in the sector by long range means, so that Sulu only observed her by sensors or a short range beacon or something. But in such a case, one'd think Sulu would say something... "They appeared out of nowhere, Sir!", for example.