We do not really have rules on how to set the grid. I would say that choosing the appropriate grid is a balance between the research question and waiting time (computational power).
Boundaries of the grid: When my research aim is to identify a minimal signature for prediction only, a small grid is obviously preferred (up to 10 or maybe 20). When I also want to interpret biology (e.g. by looking into protein-protein interactions), I use a larger grid (up to 50, 100 or even 150 in rare cases).
Fine vs coarse grid: This is a matter of how long you are willing to wait. Tuning the sPLS-DA is not really a demanding task, so I always use a very fine grid to get very precise results.