"Small question" or "little question"?

Solution 1:

Little is a very subtle word in English. A little thing can sometimes be a very important thing, as when Boromir and Frodo confront each other over the One Ring.

Asking a little question or just one small question can sometimes be a slight self-deprecation, often used by emotionally-aware leaders https://www.inc.com/anne-gherini/what-a-self-deprecating-sense-of-humor-says-about-your-eq.html

However, it can also be used to signal to an audience that something of importance is at hand. Anyone who enjoyed the detective show Columbo would have recognized the writer’s use of this technique. https://en.m.wikipedia.org/wiki/Columbo

So to answer the OP, it’s fine to use little question or small question in this context, bearing in mind that the adjectives can sometimes carry a larger meaning.