Hello,
You cannot really specify that, you just give the discharge and it will be entered on one point with its own volume around, so supposedly when we have a source it has a dimension smaller than the mesh size. However, if you specify velocities at the source to take into account the momentum entered by the source, it will be the discharge divided by the section at the exit and here your dimensions will have an influence.
With best regards,
Jean-Michel Hervouet