I think a good starting point for a mini setting is as follows:
1. Factions (and how they work, or don't work together ie realtionships).
2. 6 - 10 well fleshed out NPCs
3. Quest ideas
4. A good overland map
5. Places of interest
6. random tables for monster encounters/loot/random events (or hexes).
7. A few smaller dungeon maps/smaller hex locations.
As far as the amount of content and pages, I think erring on the side of "leaving them wanting more" is a good place to start. Being clear and concise with the above bullet points (2 paragraphs?). As far as the amount of pages, once again I wouldn't set a limit per se but enough to have the setting fleshed out.
As far as content and the idea for a mini setting is concerned, there is more than enough of the "typical fantasy" settings out there. (my favourite is Thunder Rift). That being said, go ahead and create your heartbreaker "typical fantasy" setting if you like. The idea of pushing the boundaries a little bit maybe more marketable (but could also fall flat). This is where I'd like to be. I had started a mini setting a few years ago called "Crimhuck" which could use a bit more fleshing out. It was a barbaric land, very swords & sorcery but with aliens. It seems that people are attracted to weirdness.