MTU change gets an error

[magicmouse]

I have my Pfsense box configured as a transparent firewall, so it is merely operating as a filter with no NAT. I noticed that UDP packets that were originally larger than 1500 bytes (which come in as a 1480 and 38 byte packet) are being discarded by PFsense, as my MTU is set at the default of 1500. So i went into the web configurator, thinking it would be easy to set the WAN MTU to some nice big number like 7500, but when I tried doing that I get an error:

[zone: mbuf_jumbo_9k] kern.ipc.nmb.jumbo9 limit reached
igb0: Coult not setup receive structures

i also tried using ifconfig on the shell command line and that doesnt work either.
Maybe  i don't need to set the MTU, maybe there is some other parameter inside PFsense. I am not receiving packets larger than 1500, but as a firewall, I gather PFsense is recombining the two packets which do total more than 1500…. anyway I am stumped and the book doesn't mention this, nor do I see other helpful information about why pfsense is discarding these totally valid packets, which just happen to be fragmented.