For my uses, fork cages and strapping a drybag to my rear rack, I went with 20in and even got a pair of 24in for larger volume bulky stuff. I've found it fairly easy to neatly tuck excess under the strap and this stays in place and never flaps around - but this doesn't bother me and I am loathe to cut a perfectly good strap that I have sometimes found super handy for other uses in life.

I guess if it really bothers you to have excess, and you'll use these just for your Carradice, then cut away - although there are darker colours that even with some tucking under, wouldn't stand out so much.
I know there are tan and olive green type colours/earthtones available that would fit well with the tones of your Carradice, and a certain amount of tucking in for me wouldnt be an issue, but that's up to you.

I even bought a really short pair, and have used that length to keep a small fuel bottle securely in a third bottle cage under the frame, using some foam to fatten up the fuel bottle and to eliminate any rattles. Super secure on all kinds of bikepacking surfaces. Black colour, so very discreet.

Overall, a great strap system, so versatile and once in a blue moon in non biking world, something comes up and I wonder how I will transport or contain something securely, and I'm so glad I have a few of these straps as they solve the problem.

Oh, and the daisy chaining thing, putting two straps together for a longer wraparound, works very well too.