hai yunusa,
generally for 0.3mtr dish gain is around 32dBi.For 0.6mtr dish it is around 37dBi.For 1.2dish gain is around 42dbi.when the high gain dish antenna’s used the receiving level will be increased and the link will be stable.
In maintenance view it is difficult to install and maintain 1.2 or 1.8mtr dishes.so its better to go for 0.6mtr dish for short hops.