Thanks for your comments guys, that helps alot, will do trial and error I can afford to make adjustments on the rig when the servo is installed.
As for the size of model, it is 48cm in length and I feel the rudder is adequate, especially if I set the rig up with the correct length rods etc. If you want to have a look at my pics, they are on my post (What if model too heavy?) Beginners...also.
GN