The calibration answers obviously strict rules.
And, contrary to what we generally think, it is not by the dimensions, that the size is defined, but by the weight.
The standards differ slightly as it is about rock or flat oysters.

Rock oysters

calibrage des huitres creuses
An oyster which weighs between 30 and 45 g is sized “n°5”
From 46 to 65 g, it’s n°4
From 66 to 85 g, it’s n°3
From 86 to 120 g, it’s n°2
From 121 to 150 g, it’s n°1
From 151 to 200 g n°0

Flat oysters:

calibrage des huitres plates
The size 5 is an oyster which weighs from30 to 40 g
The size 4 is between 40 and 50 g
The size 3 is between 50 and 60 g
The size 2 is between 60 and 70 g
The size 1 is between 70 and 80 g
The size 0 is between 80 and 90 g
The calibre 00 and the calibre 000 correspond respectively to 90-100 g and 100-140 g.