How can we change mtu size in cisco router?

You have to run the following interface subcommand to change mtu size on a cisco router :-

router(config)# interface fastethernet 0/0

router(config-if)#ip mtu ?

<68-1500>  MTU (bytes)             (you can configure any value between 68-1500 )
router(config-if)#ip mtu <>

Thanks