Learn English – How to say “a small and not so important question” concisely

As indicated by narx's answer to a similar question on ELU (English Language & Usage, ELL's "sister site" aimed primarily at linguists, etymologists, and serious English language enthusiasts), only little can be used as an unqualified noun to mean a small amount (where adjectival small needs an explicit noun, such as amount or quantity).

The other interesting point made about little there is that in...

1: I have a little experience teaching English.
2: I have little experience teaching English.

...the two sentences have almost opposite meanings - #1 implies you have at least some experience, whereas #2 implies very little (perhaps almost none). In fact, #1 is often used somewhat facetiously to imply you actually have a lot of whatever is being spoken of.

Apart from the above distinctions, there's also (as kiamlaluno points out) the fact that little often carries more "affectionate" connotations than small. And from BBC Learning English, little is more common in the sense not much when coupled with abstract nouns such as hope, chance, change, effect, use and point.

In most other contexts, there's little to choose between the two words. Use whichever you like.

This question is related.

Elaborating (though I don't think that any more needs to be said), both small and little are fine to say.

The sentence is not using small or little as quantifiers, but rather to denote the size.

However, it is perhaps worth noting that little question is more popular.