for a 50cc airplane,will you choose a 13kg digital servo to cover the plane just right or prefer a 20kg one?

13kg servo could be ok.

Is this a Cub or an Extra? Is this a rudder servo? Does it have a split elevator or ailerons?

What Ahicks said - plus is this servo for the rudder, elevator, aileron, throttle?

We need to know the model, or at least style, and the application.

It‘s a 50cc extra, 5 servos are needed. I'm considering the servo for rudder.

Single servo on the rudder I'd go with something like a HiTec HS-7940TH (181 oz-in at 6.0v and 222 oz-in at 7.4v)