Thanks for all your answers. Today I did some test just to see what would represent different dot size at 100 yards, 50 and 25. I first started with a one inche red dot (1MOA) that I glued on the target. Well, seeing that at 100 yard is not too difficult if there is a good contrast between the target and the dot. But more difficult if the target does not offer a good contrast. Just to resume all my test I should say that things become a lot easier with a dot of 3 to 4 inches at 100 yards. I also discover ( I should have seen that long before) that the front sight roughly cover 4 inches at 100 yards. Also I think for close range a bigger dot, may be around 8 moa (about 3/4 of an inche at 10 yards) should even be better. At first I tought that a 4 moa dot might be too big but now I think it is a very good comprosise.
Have a nice day,
Clovis