vSphere HA slot sizes are used to calculate the number of VMs that can be powered on in an HA cluster with “Host failures cluster tolerates” selected. The slots size is calculated based on the size of reservations on the VMs in the cluster. HA Admission Control then prevents new VMs being powered on if it would not leave any slots available should a host fail.
The slot size for a cluster can be seen by going to the Summary Page for the cluster and clicking the “Advanced Runtime Info” link in the HA box.
If none of the VMs have CPU or RAM reservations, a default of 256MHz and 0GB is used.
The slots per host is derived by taking the total available CPU/RAM for the host and dividing by the slot size. Some CPU is reserved for the system so it will usually be a little lower than the full amount. So a host with 2xquad-core 2.4GHz CPUs (total 19.2GHz) and no VM CPU or RAM reservations has 73 slots and will only allow 73 VMs to be powered on if the cluster has two hosts and is set to protect against a single host failure.
Obviously this allows a very minimal amount of resource for each VM, so either reservations should be set for each VM, or slots size can be manually adjusted (see the VMware vSphere Availability Guide (pdf) for full details).
Note that the slot size is used for admission control calculations only. It has no direct effect on the resources available to VMs should an HA event occur.
There is a VMware Knowledgebase article (1010594) which has some details of the difference in VI3 and vSphere 4.x.