Only time I've experienced drone in my cars was:
1. 3" system with Flowmaster 40s with before-axle turndowns - 2.5" tailpipes cured it.
2. '64 Savoy wagon 3" system with almost anything - tried 3 different sets of mufflers to get the right sound - and tailpipes cured that.
Both had quite "healthy" RBs that seemed to merit big pipe.
I think the interior volume on the wagon - basically a big drum - was a large part f the problem.

As for case length, it seems to me that the more volume in the case, the less restrictive it is.
But I have no date to prove/disprove that.