Seb,

I would put weight in the fact that they aren't providing you with specific guidelines. That means that the content of your portfolio will be much more important than the manner in which it's presented.

This, of course, doesn't mean you can get away with throwing a bunch of pics in a shoebox, but rather that you shouldn't get too worked up over the physical packaging of the work.

You may be able to get some more info from someone in the department you're trying to get admitted to, but I agree that generally, you probably should include a variety of work. I don't think a school would expect you to have a precise and distinctive style at this point, but rather they're looking for a good eye, basic craftsmanship, creative ideas, and artistic potential.

Of the sizes you list, I think the larger (you mention 8x12, but also 8x10) would be better than the smallish 4x6. Again, depending on how large a body of work you have, normally 20-30 prints or so should be enough. And better 20 good prints than padding it to 30 with 10 extra mediocre ones.

As for binders, I don't think that will make a difference at all. Just make sure you get one that won't fall apart...