I have the Retro 5 and easily fit 2 M bodies, 3 lenses, and hoods, filters, etc. It's been my main bag for the past 2 years and I love it.
I just picked up a Rolleicord V and I know that when I travel or go on long shoots I will want to take the 'Cord and a couple of M bodies (or an M and a Barnack, which isn't a huge size difference), plus lenses, etc. I usually only have one lens mounted on an M at one time. I am considering picking up a larger Retro bag, but am not sure if the 10 or the 30 is the better option. Anyone have any opinions from experience? The 20 is too tall for my tastes. I suppose I could order both the 10 and the 30 and try them out, but short of that, I thought I would see if someone here would have an idea.