You can start with the least conservative approach and choose the largest cell size that you have listed (your delta x). Once you have established your time step, run your simulation until you have somewhat of an established flow. You can then plot the Convective Courant number on a relevant surface (you can define a plane for this) and see if this meets your requirements. Based on this, you can then decide if you need to reduce your time step or not.
Another question, you mentioned that the region of interest is surrounding blades. Are these blades moving with a Dynamic Mesh?
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