The time step and grid size should be chosen to ensure a numerically stable solution, the stability criteria are discussed in the MODFLOW and MT3D manuals.

Look up Courant number and Peclet number. That's a good starting point to set your grid size.

-you can depend on peclet number and this number should be less than certain number depending on the spatial discretization.

If your peclet number is less than that certain number your model is ok. Otherwise you should make your spatial discretization finer.

In addition to all of the above answers, the grid size is also depending on other factors:

-How precise and detailed the solution wanted?

-How detailed is the input data?

-model domain and the computing capability

I agree with Michel answer