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A VoIP WLAN (wireless) subnet? For wireless, subnet sizing might differ from wired LANs as different media considerations apply.
For wired, you're correct, VoIP only hosts tend to be a little easier on the network than data hosts. As such, it's generally okay to go with a larger subnet, i.e. a /23 might be fine.
One "gotcha" though, often VoIP phone hosts share the same physical port as a data host. Even when they use a dedicated VoIP VLAN on the edge port, the port is physically sending data broadcast packets down the port. In shared port environments, you generally don't want to increase the size of the VoIP subnet as you might with ports dedicated to just VoIP.
PS:
Giuseppe is correct, placing VoIP subnets into their own VRF may improve VoIP security, but it also raises complexity. You might want to carefully consider how secure you want your VoIP devices to be and the related cost to achieve. A "half way" approach might be segregation of data and VoIP into different VLANs and subnets (all in the same routing topology, though).
