3-4" parer, 6-7" petty chef, and 8-10" chef
in my kitchen:
parer gets used primarily for opening packages -- i like to use 1/8" stock but with a wicked distal taper, make sure the point can easily slide into seams but not be fragile.
petty gets used the most often for everything from making sandwiches, slicing onions, peppers, tomatoes to slicing chicken breast -- thin flat grind to .020 behind the edge and then convexed to zero, put a fresh edge on with a water stone.
the chef for larger items like breads, pizzas, melons, squash etc -- I grind this same way as petty but perhaps with a bit more convexity or maybe even a hollow s-grind.