Roger,
I bought a few months back one of your changing bags (it is very nice!!), I use it for changing 5" x 4" darkslides and have no problem at all. Its just a case of practice and being organised, of course the bigger the better but the one I have does me fine.
On the other hand I bought a Patterson 27" x 30" for changing 10" x 8" darkslides and although it gets me by, it is hard work and a larger size would make life so much easier.
One thing that occurred to me having used both bags is that a bigger bag with an internal frame to form a 'chamber' giving freedom of movement within might prove useful, how practical this is to do I'm not sure - well it was only a thought.
Cheers
John