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