Hi baggio,
calculating the correct value of transformers is the job of those who produce them.
You plan to use 12V in and 220V out. Your design is a pure square wave inverter, so don't connect loads which are not designed for square wave AC.
The desired power output is 20W, which is 1.66A at 12V. Reverse a 9V/2.5A transformer with a center tap on the 9V-side. The first losses involved are the forward voltage drops of the 2N3055 of 0.7V. The second losses occur by the alternating magnetic field.
So using a 9V transformer will bring you pretty close to the desired output voltage.
Boncuk