Growing up in Minnesota we always said things like "Oh, for sad!" or "Oh, for cute!" which mean "oh, that's really sad/cute" or "oh, that's too sad/cute". I wonder if that usage has its roots in the Scandinavian heritage we have in Minnesota? Either way, the usage of 'för' in this example made me think of home and smile.
That's an interesting point, but I guess you'd need to have an another one word following 'små' (persons or people). I'm not sure though.
You're right, in theory it could work, but we don't tend to use 'små' as a nominalized adjective very often, especially without an article. So in practice it doesn't work in this case.
Um, I just said that you can tell here because we don't really use små as a nominalized adjective. If it had said Byxorna är för små barn, you could have been sure it would have been 'The pants are for small children'. So as a rule of thumb, when it's för + just an adjective, you can assume it's too, but when you have för + adjective + noun, it'll be för. (This is a bit simplified and won't cover every case, but it's a start.)
They could be too small without being very small. They may even be very large but still too small.
why is "små" here an adverb? it is an adjective in English, is it something about the preposition that it becomes an adverb in Swedish?
It is an adjective here too. But the adverb taught in the sentence is för in the sense of too.
Generally, because it precedes just an adjective, rather than preceding an adjective + noun. See the other comments for detail.