Max MTU size depends on your ISP's settings. You can find your max MTU by pinging your ISP with different size packets.
First you'll want to raise your MTU to something super high, so you know your settings aren't limiting it. According to
http://www.linuxnewbie.org/nhf/intel/modems/modemtweak.html you can do that with "ifconfig eth1 mtu 5000". Just "ifconfig" will tell you which interface is which, and the current MTU.
Then use "ping -s ####
www.isp.net". That will send a packet of #### bytes to your ISP if it's not too big. You want to keep trying that until you find the biggest packet you can send to your ISP. There are 28 bytes of overhead, so the biggest ping you can send with a MTU of 1500 will be 1472. You want to keep trying bigger pings until you find the biggest you can send. If it maxes out at 1972, then you know that your ISP uses an MTU of 2000.
You want to set your MTU to the same thing so the packets don't have to be resized. (Like if you packed something in size 5 boxes, then the next guy had to break up your size 5 boxes and repack the contents in size 6 boxes to ship out.) Like the first step, you can set the new MTU with "ifconfig eth1 mtu 2000". I'm a Linux newbie though, so I don't know if that's a permanent change or if you'd have to edit something else...
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