Sprinkle a small amount of coarse salt, such as kosher, and once again using the flat side of the knife, rub across the garlic using the salt to break it down into smaller peices. If you account for the salt from your recipe, you will not risk making something too salty to eat.
You can continue to work the garlic until it is the desired size, even making it into a paste. You might want to "rock" the sharp edge of your knife through the garlic if you see any pieces that are over sized and out of place.
Now you are ready to go and make some wonderful garlicky dish that will be sure to please!