Having the right grind size for your brew method and recipe is key to a good cup. Grind size is all about proper extraction by increasing or decreasing the surface of the ground coffee. Finer grind = more surface area = more extraction.
Check out this post for more info about adjusting the variables that go into coffee brewing.
Dialing in your grind size to get the best extraction usually takes a bit of trial and error. But its is nice to have a starting point to limit the guess work. Below is a chart of the different grind sizes with references to help. Also, if you use the Porlex Mini, Made by Knock Aerspeed or Aergrind, or VSSL the chart includes settings for those hand grinders to help get you in the ballpark.
The ideal goal when grinding coffee would be to have the exact same surface area on each of the coffee grounds. But a grinder is essentially shattering the coffee bean. So we end up with some smaller "fines" and bigger "boulders" whose surface area is different. A good quality grinder will have good, consistent burrs and limit these outliers than can throw off the extraction and thus the taste in the cup.
The Porlex mini adjusts the grind size by turning a knob on the bottom of the burrs. There are no marked references but it has indexed notches or clicks that you can feel while turning. The chart references setting based on these "clicks." The Porlex is a durable and affordable grinder that fits inside an AeroPress for space savings.
Made by Knock Aerspeed
This is a fantastic grinder whose quality is immediately apparent once you pick it up. High-quality, machined construction, tight tolerances, and fast easy grinding. Grind size adjustments are by moving the handle and cap with a very clear and easy to use numbering system on top. Bonus feature, the handle notch can be used as a bottle opener!
To set, turn the handle until the burrs are fully closed (no gap). This is your 0.0 position. Now turn the handle clockwise while holding the cap to open up the burrs. As you pass the "12" position you are now at 1.0. Pass "12" a second time 2.0, third time 3.0, etc... For example and AeroPress grind setting of 1.7 would be full turn to the "12", then a continue turn till reaching the "7" mark as pictured.
A high quality and lightweight grinder that is packed full of innovative features in a nice looking color scheme. Like the Porlex, grind adjustment is done on the bottom side of the burrs and has tactile "clicks" that you can feel. The VSSL goes a couple steps further and adds reference numbers along with coarse/fine direction indications. Like the Aerspeed the number system starts over every time you pass "8"; 0.0, 1.0, 2.0, etc...
The handle extends for extra leverage to make grinding that much easier. And when not in use can be folded over and screwed down to stow or used to clip to a bag. The whole body is made of machined aluminum with a tight and smooth fit, including the catch up that securely screws into place. And the cap on top securely clicks into place and is easily removed with the press of the button in the center.
The only real downside I see to this grinder is it is just a bit to large to fit inside of an AeroPress. So it will potentially take up a little bit more room inside of your coffee kit.