Counter Depth: The standard is 24". But again, this can vary based on your size, appliance depths, and details of the backsplash and cabinetry.
Upper Cabinet/Shelving Clearance: The distance from the countertop to the upper cabinets or shelves should be 15 - 20". Any lower and you'll impede on work space. Any higher and it's going to be tough to reach uppermost shelves.
Kitchen Triangle: Remember the kitchen work triangle? For maximum efficiency, the sum of its three sides should not exceed about 276 inches.
Circulation Space: Have a galley layout, and island, or a U-shaped kitchen? The circulation space between countertops should be at least 48". (This standard seems large to us, but we're accustomed to a tiny apartment kitchen, where this space is typically much smaller). Don't make the space between counters more than 64", however, or it will simply be too big and you'll lose a lot of efficiency during meal prep.
Stoveside Counter Space: Try for at least 12" of counter space on at least one side of your stovetop. Same goes for the refrigerator, preferably on the open side of the door.
Now, in the name of small and cool, let's talk real numbers for small kitchens: Our smallest apartment kitchen had more like 30" between counters, 4" of counter space next to the stovetop, and no counter space adjacent to the fridge. Somehow, though, you make it work and even come to love working in such close quarters. Let's hear your kitchen dimension experiences and preferences: small, large and anything in between!