Useful article discussing LUN sizes with VMware ESX
Size of the Virtual Hard Disks
Size of the RAM for Swap
15% for SnapShots
10 – 20% Free Space for log files
Personally I would only have 10 – 15 Virtual Machines per LUN, Keeping extremely large / heavy use machines separate. Use RDM’s for things like large SQL / Exchange Stores as you then then make use of SAN advanced features.