Translation:This bed is small.
I've seen it explained on other sentences that は means "As for the thing," but I've also heard it explained that は indicates the concept of things in general, while が indicates "a thing" or "the thing".
When you say が means "the point of the sentence is to say something about what is small..." what does that mean, exactly? How would you translate 「このベッドが小さいです」?
If I wanted to say something about the bed that is small, I would use, uhh, something like この小さいベッドは安いです。
Sorry for the late answer, but what I meant is, for instance : imagine you're looking for something small (for whatever reason), and then you stumble upon this bed. Then you'll use が, because the point of saying "Well, this bed is small" is to point out something that is small.
English does not make that kind of difference, so we would say "This bed is small" in both cases, maybe with a different emphasis on whatever word we want to highlight.
Kore = This (a noun) Kono = This (an adjective)
For example: Kore wa beddo desu = This is a bed
Kono hito wa gakusei desu = This person is a student
Then there's Koko, Soko and Asoko, which indicate place. Koko wa toshokan desu = This place is a library.