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.
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"All that is gold does not glitter,
not all those who wander are lost..." J.R.R. Tolkien
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.
I use 3/4" double thickness (1/8") butyl tape purchased from Vintage Trailer Supply. This size works well for resealing jalousie window frames. And its the correct gooey sticky composition.
The butyl tape at room temperature should be sticky like chewed gum so that you can stretch it a foot or two before it breaks.
Trailer: Sold the Bigfoot 17-Looking for a new one
Washington
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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.