It's generally accepted that a solid medium grind is best for pour overs. The general spectrum on grinding goes something like this:
Finest - extraction via pressure (Espresso)
Medium - extraction via filter (Drip/Pour Over)
Coarse - extraction via immersion (French Press, Cold Brew)
It's also important to know that two of the other key aspects of extraction are time and temperature. That said, beyond fine tuning your grind for your pour over gear of choice (Chemex, Dripper, etc.), the best investment you can make in coffee gear is your grinder.
In other words, if you're still grinding with a blade grinder, and you're ready to fine tune your grind, it just might be high time to upgrade to a burr grinder.