minimum area of neighborhood required?

Pete, If I understand the first part of your question, the answer is no. To achieve the credit, the LEED project needs to be within 1/2 mile of a single residential development of 10 units per acre. So, a single apartment building with a 1 acre footprint and 10 units would satisfy this credit requirement. However, if no such building exists, the requirement can be met cumulatively with a mix of multi-family and single family buildings, so long as 10 units per acre exists somewhere within the 1/2 mile radius. For the second part of your question, I haven't worked on a project that successfully achieved the credit using a student housing building to meet the density requirement, but I don't see why that wouldn't qualify. Based on my experience, that meets the intent and should work.

Same question as above... would like to know if there's an update about dorms qualifying for the high-density housing requirement. They are indeed "residential units..."

I have achieved this credit using a residential dormitory as a means of compliance. This project was on an urban campus so the local zoning applied and we used that to illustrate our case. This dorm would not be considered "on campus" per se but the reviewers gave us no impression that the dormitory didn't qualify. I know this is not a direct answer to your question but I agree with Steve - dormitories meet the intent by definition.