in a nutshell:
put video and voice into separate subnets, for QoS policy easyness. stick your servers in a separate subnet, for the same reason and possibly to enable the ability to differentiate security policies based on their subnet and also to trim broadcasting from other subnets.
size? depending on the number of hosts, so make sure you use subnet masks that allow expansion.
If you are merging, then you will need to bite the bullet and you will need to re-ip some machines.
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