2 MB, the limit was considerably smaller than I thought. You're probably right on this, since I can't think of a whole lot of worthwhile Skyrim mods less than 2MB in size, but that's neither here nor there -- Nexus only requires you to register an account, it doesn't require an added step of registering your game.
Also, I am sure that EQOOE and myzael would like an answer.
I think entering a one time code that does not personally identify you and only unlocks additional features and access on the site and is readily accessible from the steam console is not that much of additional effort. Not for the vast majority at any rate.
And Answered in the post above this one.