How large of WiFi VLAN before broadcasts are too much?

My building currently has about 12 Cisco 3702 WAPs on each floor, controller by a Cisco 5520 WLC. I have each floor's wifi in its own /23 vlan, but I am finding that it doesn't seem to be enough anymore. I am thinking about expanding the VLANs but my concern is with broadcast traffic bogging down things if the network gets too large.

Should I split the building in half and have two /23 networks per floor? Or would I be ok do a /22? Option A wouldn't be much more work than just expanding the vlans, but would it even be necessary? Plus, if someone walks to the other side of the building for a meeting, they would need a new IP when joining a new AP group.