To set the MTU size on a Draytek 2820 series router the following steps should be performed.

1. Telnet/SSH into the router
2. Using the command 'wan ppp_ms' select the size you wish to use
     
     >wan ppp_mss 1500

3. To changes use the command 'wan ppp_ms ?'

     > wan ppp_mss ?
     % wan ppp_mss <MSS size: 1000 ~ 1500>  
     % Now: 1500