I just remembered to go and measure my mufflers/silencers. I love a bit of muff-diving, me!
In English, the measurements are as follows.
Drive side:
A = 23 7/16". B = 8 1/16"
Timing side:
A = 24 1/8" . B = 8 3/4"
The difference is explained by the fact that I had to buy a replacement silencer after one developed a loose baffle and started to buzz loudly at certain revs.
The downpipes I have go a long way into the silencer, maybe 4 inches. I don't know if that matters. The exhaust sounds quite nice and throaty though!
Additional: The place where the stand foot sits on your silencer is much nicer than mine, which are just dented depressions.