There are various sources that attempt to give rules for adjective word order based on semantic categories, for instance here, or here or here or here, which are broadly in agreement. (However for contrast here is another source which uses different categories).
- Quantity/Possessive/Demonstrative (e.g., ten, several, my, your, this, those)
- Observation/Quality/Opinion (a catch-all category, e.g., soft, ugly, difficult)
- Shape (or Age instead)
- Age (or Shape instead)
- Origin/Nationality/Proper-name adjective (e.g., Greek, lunar)
- Material (e.g., wooden, metal, plastic)
- Purpose/Qualifier/Type (often closely tied to the noun, e.g., sleeping bag, swimming pool)
These rules may be helpful, but seem entirely empirical and not always clear-cut. Note that the different sources don't quite agree on the semantic categories, and differ for instance on whether "shape" comes before "age" or vice versa. (You could plausibly construct sentences that require one or the other, for instance "ancient oval amulet" vs. "rotund young man").
For what it's worth, I personally would say "small dark over-crowded bedroom", but I couldn't really explain why, other than shrugging as native speakers are wont to do and simply saying "it sounds better to me". This isn't really consistent with the links cited in the first paragraph, unless you try to interpret "over-crowded" as a type of bedroom rather than as an observation or opinion about the bedroom. Maybe the multisyllabic "over-crowded" gravitates naturally next to the two-syllable "bedroom", although the cited links doesn't mention syllable count or prosody as criteria. So I'd say the empirical rules have their limitations and can't always be relied upon.
"Small" sounds better to me than "little" here even though they are synonyms. "Little" is sometimes used figuratively in a diminutive or affectionate sense whereas "small" can be more literally interpreted as pertaining to actual size. Also "a little" + adj. could be reinterpreted as "a bit" + adj. in other contexts, (i.e., "a bit over-crowded" or "somewhat dark") and maybe this leads me to avoid using "little" here.