I have a simple antenna questions with maybe not so simple answers. Please help.

1. Between two 5.4Ghz GRID parabola antennas with both having 21dBi gain the better antenna is the one with bigger grid?
Does the fact that they both have 21dBi gain make them comparable in performance?

2. Patch antenna vs grid parabola antenna (both having the same gain).
Is it OK to say that one performs better than the other (bot having the same gain)?

3. Does the size of the patch antenna panel have any influence on the performance (SNR ratio for example).
Thank you for helping.

1. Between two 5.4Ghz GRID parabola antennas with both having 21dBi gain the better antenna is the one with bigger grid?
Does the fact that they both have 21dBi gain make them comparable in performance?

Exactly, same gain means exactly that, will get exactly the same signal strength, no matter how the dish looks.

2. Patch antenna vs grid parabola antenna (both having the same gain).
Is it OK to say that one performs better than the other (bot having the same gain)?

Again, same as above with biggest difference the wind load.

3. Does the size of the patch antenna panel have any influence on the performance (SNR ratio for example).

Not necessarily, although not all antennas is created equal and unfortunately the best way to test them in a specific situation is field test.