If the APs are in the LAN the default should work.
If the APs are connected via a WAN/VPN with reduced MTU you'd need to use the minimum MTU which is supported on the link.
I.e. I've a AP3825
@home = cable modem provider and I've a VPN tunnel with a firewall to the HQ.
In that case I've used 1300 as my DSL and VPN take away from the max. of 1500.
Normaly the APs should discover the MTU that is supported on the link BUT if the devices between are not setup to support MTU path discovery you'd need to set a static value on the AP.
Please refer to ...
https://community.extremenetworks.com/extreme/topics/access_point_path_mtu_discovery
https://gtacknowledge.extremenetworks.com/articles/Solution/IdentiFi-Wireless-AP-s-do-not-have-backu...
You'd change groups/all APs via AP multi-edit in the GUI > AP > Bulk Configuration > AP Multi-edit Settings
-Ron