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As the other posters have noted, a /24 often is a good starting point for a maximum subnet size. Occasionally it's too large, and conversely sometimes it can be pushed larger. The determinate is broadcast volume.
On something like Cisco's WLC and/or WiSM, you can often safely push client subnets larger than a /24 because there's not the same broadcast issue between wireless clients on the same subnet. If your have "temporary" wireless clients, you might also want to use a much larger subnet to allow for a larger DHCP pool, as IPs can be tied up, with transitiory clients until their lease expires. (Of course, you can also decrease the lease time to recover inactive IPs faster.)
Recently, I've been involved in a project where we've been migrating multiple internal Wireless (WLC or WiSM) subnets (all same SSID and security) to a single /22 or /21. So far, we haven't seen any problems, although rarely more than a couple of hundred active wireless client at anyone time. Definitely, a little easier to manage.
