Hello,
This is a question which is probably not meant for Telemac-2D, but rather for the mesh generator BlueKenue. Anyway by giving this kind of parameter to the mesh generator, you may have different mesh sizes, it is only an average.
If you want to test different resolutions, it is advised to use stbtel, that can divide every element in 4, you then divide the time step by 2 to get the same Courant number.
With best regards,
Jean-Michel Hervouet