Largest leather strop?

I would just buy a piece of leather and cut it to the dimensions you want. I've made a few that are 3 inches by 12 inches.

 

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For larger knives, I'd personally focus on more strop length, and not worry so much about the width over about ~ 2.5" - 3" or so. I've always noticed that even when I tried stropping (of any length blade) on a very wide surface, not much contact is ever made between knife-edge and strop past about 2.5" from the handle/bolster end of the knife, when it's flush to the edge of the strop. Any blade with any curvature at all will leave most of the edge elevated over the surface, to either side of the portion being worked, which is usually quite narrow. I've also noticed this in grinding/sharpening/polishing large blades on full sheets of sandpaper. You can see on the sandpaper or on a loaded strop where most of the work gets done, by the loading & blackening from swarf on the surface. There's seldom much blackening past that ~ 2.5" mark. Length approximating arm's reach of maybe 18" - 24" of stropping surface works well, at a width of maybe 2" - 3". A sweeping, heel-to-tip stropping pass is how I use a large strop like this, and the length will allow such a pass with very large blades.

The one advantage of a wider strop might be if you choose to use it from both sides, if one edge loads up with excessive swarf. But that's a minor advantage anyway, and a leather strop is relatively easy to clean up when it gets loaded with swarf.