Prior to using TCP Optimizer (site is down for everyone at the moment) the router should be set to MTU=1500 - the computer's subinterfaces are default set to 1500 so there's no need to reset them for the program to work.
Until the site is up and running again, then just use TT's recommended or you could use 1500 but to change the subinterfaces, they are done with netsh cmds as an administrator from the cmd prompt.
You can also check the optimum setting yourself from the cmd prompt with ping cmds - this Kitz article explains
http://www.kitz.co.uk/adsl/MTU2.htm
From Start type
cmd then right click on cmd and select Run as administrator - Yes - then enter these cmds :-
First, to view the subinterfaces enter -
netsh interface ipv4 show interfaces
This will display the Pseudo Loop, the Wireless and Ethernet (LAN) and in the left column will be the Index Nos for each.
Say they are 15 for the Wireless and 12 for the LAN -
To change the MTU value for the Wireless (15) to 1432 enter this cmd -
netsh interface ipv4 set subinterface 15 mtu=1432 store=persistent then repeat the cmd for the LAN using its Index No.
Redo the ...show interfaces cmd to confirm the new values then type and enter
exit to close the cmd window.