Gotcha, clear now.
The pair of 20 inch ones that I had were a bit short for some bulky things, so daisy chained them a few times to get around a fatter dry bag. I ended up giving them to someone, so got another set of 20in but kind of guesstimated that 24in ones could be handy , so took advantage of a sale at a ski shop when I got those and some real short ones.
I get the anti-spending-money-on-length-I'll-never-use thing, makes sense. I guess you'll have to put some bulky things you think you might stick on your Carradice and use a cloth measuring tape or just some strap to guesstimate a logical length that will be useful--kind of hard for us to comment on that. I recall doing that with some regular old black nylon straps I have, and measuring sort of what would go around X or Y thing.
One thing though, with soft things like drybags or whatever, voile straps really will compress stuff, so there have been times I think my 20in ones are too short, but then when just being able to get the end of the strap through the tab thing, I can actually cinch the said thing down a lot, and all of a sudden, I have a ample strap length.

sometimes too though, you can position the tab thing closer to your Carradice, and then tuck stuff under where its not sticking out like a sore thumb.

the main thing with voile or other similar material straps, is how impressive and reliable Ive found them to be, easily cinching them down securely, and not loosening or undoing when I've been pretty much been riding on mountain bike trails.