DESGRIDSEARCH_IMAX is number of cells, not cell spacing, as the documentation indicates. Spacing is the reciprocal of number of cells, as you see in the code.
To avoid missing collisions we suggest lowering Max steps between neighbor search (neighbor_search_n). It is not usually necessary to modify DESGRIDSEARCH_[IJK]MAX. See DEM Particles with unrealistic high velocity - #12 by gbotha
A “thinner” mesh generally means finer grid resolution with more cells packed into the same volume.