Some groups follow a strict, first come first served approach with the rationale that the sooner a person invests money the greater risk they are taking. Another approach is to have everyone place yellow post it notes with their name on their first, second, and third choices on the site map. These are things you want to keep but don't use more than once or twice a year. Things like Christmas ornaments, camping gear, boxes of memorabilia etc. A high ceiling can make a small home feel much bigger.
Skylights do wonders for a small space.
When you are planning units be sure you understand very clearly what size dimensions actually mean. One group set up a sizing station on their site, with several stakes and string and tape measures. They then actually physically measured off spaces and got inside them to understand what a 10 foot by 8 foot bedroom felt like. They also cutout cardboard templates for beds, couches and tables and placed them in the space to get a real feel. You can do this on scale paper also.