there is no standard size really. Most lenses are different sizes. But if you buy a stepdown ring that's a pretty good idea so you can use a bigger one on a smaller lens.
I can't imagine a polarizer on a 10mm lens to be very successful.
its supposed to have some cool effects.
With a wide angle lens like that, I would definitely recommend a slim profile CPL. I got a B+W CPL for my 12-24mm and I have not had any issues with it screwing up the scene.

Three problems/concerns though should be mentioned about it. It does not have front threads on it which means I can't screw another filter on top of it (no biggie), I can't use me lens cap (an issue) and it's expensive.

On the plus side, it's freakin' awesome.

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