Mesh size for dynamic analyses

[StoyanAndreev]

Considering the Kuhlemeyer and Lysmer criterion to transmit a maximum frequency fmax the mesh size should be L<C/(10fmax), where C is the wave velocity. My question is if I am using second-order finite elements (8-node rectangular for example)what is L - the size of the FE or the distance between two nodes on the element's edge (about half of the element size)? My idea is to optimize the DOFs and the solution time by using coarser mesh with 8-node FE if possible.