We will be implementing a VLAN - with the primary goal being to limit broadcast traffic. My question is - how large should each individual VLAN be? Most of the workstations would be Windows XP (around 80%), some Vista, some Windows 95/98 still around.
Will a /24 (for example 172.16.x.x/24) for each VLAN be ok? Or should I make it larger or smaller? The Windows workstations will be accessing the Internet, and local fileservers, local Oracle/SAP servers.
The rule of thumb I usually work to is if the network consists of entirely IP hosts (most of the time...) then go no bigger than a /23 (i.e. max 510 hosts), if you have other protocols running in addition to IP then go no higher than a /24 (254 hosts). However with this in mind you need to consider the network topology, STP diameters, fault footprint/isolation and redundancy.
It is generally recommended to keep VLAN's local to a switch, i.e. don't span VLANs between access switches. Doing it this way keeps fault STP simple and loop-free, it keeps potential fault footprints isolated (broadcast storms etc), allows easy troubleshooting (you know where your IP subnets are physically) and is easy to grow due to it's modularity.
Have a read of the High Availability Campus Design SRND - http://www.cisco.com/go/srnd