Hi Fabio,
As per my understanding you are having 2 cameras and 1 lidar whose field of view is 120° and want to know the size of the checkerboard to use.
According to the calibration guidelines, you need to create a rectangular checkerboard (729x810mm) that contains an even number of squares along one edge and an odd number of squares along the adjacent edge.
To know more about the guidelines, please refer to the following documentation:
Hope this helps :)
