"They will all be set to have an Object Bonus of 2 which is to say that the number of prims available per square meter is twice that of a regular Region. To handle this, Linden Lab (in the person of Governor Linden) will need to retain over half of the land in each Region. We’re aiming for at least 56%, which is typical of existing city regions."
Linden Department of Public Works Update « Official Linden Blog