Unless I do not read the man page properly the information is available
there.
NAME
ifconfig — configure network interface parameters
SYNOPSIS
ifconfig [-AaC] [interface] [address_family] [address [dest_address]]
[parameters]
...
and lower you have:
...
The following parameters may be set with ifconfig:
...
mtu value
Set the MTU for this device to the given value. Cloned routes
inherit this value as a default. For Ethernet devices which support
setting the MTU, a value greater than 1500 enables jumbo frames. The
hardmtu output from hwfeatures shows the maximum supported MTU.
...
So if you read the man page you should have the information you need.
Hope this help
Daniel
