I've bought 1/8" X 1" and it has worked well. In some cases I've been forced to cut the 1" width into narrower sizes depending on the place of application. However, I can say that this stuff has solved a lot of our leak issues, like magic.
I haven't used it, but it seems to me that if you buy it a little thicker than necessary it will squish down, but thinner than needed won't seal. Wider than needed can be trimmed away, but narrower than needed is harder to fix.
I am surprised that you haven't gotten more responses by now from folks who've been down this road.
Trailer: Sold the Bigfoot 17-Looking for a new one
3/4 wide by 1/8 thick is the only size readily available at the local Camping World. I've never run into a project that I could not do with this size. Occasionally I need 2 layers for extra thickness or 2 rows for width, but this size has done everything I need.