Translation:It is a small bed, isn't it?
"Is not it a small bed?" rolls so so well off the tongue compared to "Isn't it a small bed?" Lol
Kidding. Clearly my joke example is more colloquial and not 100% grammatically correct, and thus less formal, but the whole sentence in Japanese expresses a casualness that would be roughly equivalent to us changing the grammar in English.
Point being: we should have the option to translate more liberally at times.
From a dictionary (大辞泉), 小さい (i-ending) has a more objective perspective, and 小さな (na-ending) has a relatively subjective meaning. e.g. 小さいベッド is small in measurement, which,小さなベッド maybe not small in mersurement, but it gives a feeling of small (e.g. has a lot of things on it or the style of the bed making it looks small)
In colloquial English (California) Huh is used like right. Therefore, would you say that, "It is a small bed, huh?" is correct? I feel that the "ne" particle ending is colloquial Japanese seeking agreement. The fact that the correction sometimes only lists it as a simple interrogative is incorrect. "It is a small bed?" Is wrong.
The ね at the end of ですね is added to, hmm, sort of invite either agreement on a sentiment or confirmation of information (depending on context) from the person being addressed. In this case, it's the former, hence the "isn't it" part of the English translation.
"What a small bed" implies more of an exclaimation or even a criticism, which is not quite the tone they're going for here.
As an adjective - good question, and hard to answer. In many cases I'd say they're interchangeable, including here, though my mental picture of a "little bed" and a "small bed" would have the former be something much smaller (in fact, if you were commenting on beds in a dollhouse, say, you'd be far more likely to use 'little' than 'small'). In other cases, e.g. "a little coffee" vs "a small coffee", they're not interchangeable - the former implies some indetermine low quantity of it, whereas the latter implies a known fixed-size serving. It's relatively unusual to say "a small bit of", and likewise "a little portion" or "a little size" (vs. "does that come in a small size"?)
Because it is not a question at all in the Japanese sentence. In other words, "isn't it?" is not a perfect translation to the ね particle. It conveys the meaning of "I hope you agree" but nothing in English can translate to this particle perfectly.
Even if it is a question in the Japanese sentence, most likely you will see a か particle at the end, but still no question marks, because traditionally Japanese language does not have this symbol "?". Ending particle か makes a sentence a question.
Couldn't this be translated as "What a small bed!"? Like the Japanese sentence, it prompts others to comment on the object of the interjection, but is phrased kind of like a question if taken literally. As I understand it, "ne" can not only be used when you want someone to comment on what you said, but also as a sort of exclamation point to punctuate your emotion (such as surprise.)