MTU settings

I would like to know how to change my network cards MTU setting. Red hat 9 i'm running atm

In command prompt, type ip link set eth0 mtu value.

That's assuming your network card is eth0.

You can check it with command ip link show eth0.

will it always be set to that value then?

If you want to set it permanently, edit /etc/sysconfig/network-scripts/ifcfg-eth0, add line MTU=value there.