Before going further in investigation to buy other monitors, I would like to understand a little more.

I have 3x 1920 x1200 24 inch

I use a matrox triple.

the size which is most recommended according matrox is 1280x 1024

infact it gives the best size of the cockpit.

with the 1920x1080 the cockpit is too small.

 

Now the question is if I want to buy 3x 27 inch and would like to have the same view regarding the cockpit what size of monitor do I need if I want to use the matrox again with higher resolutions but with the same resultin view?

You could adjust your zoom to make the cockpit larger, 1280x1024 is a non-native resolution on those screens, images will appear larger but quite blurry compared to the native resolution.