Excerpts from Mobility Design Guide-
"The default behavior of the WLC is to block broadcast and multicast traffic from being sent out the WLAN to other wireless client devices."
In most of the WLAN designs it is considered safe to use large subnet sizes if you are not allowing Bcast and not much of Mcast traffic.
As long as the WLC is not propagating Bcast/Mcast traffic and no spanning tree loops, the Mobility subnet sizing is not a constraint for WLAN. Usual recommendation to Large deployments is ---no subnet size more than /20 and /19.
Also it depends so much on the type of traffic specially Bcast/Mcast traffic. So it is NOT advisable to use large subnet size in case of Bcast/Mcast/STP loops in N/W.