Baby bok choy are usually about 2½ to 3-inches long. If you only find the bigger, longer bok choy, then trim off the root ends and cut the stems diagonally into thick 2-inch slices. Put the thicker white parts in the steamer first. Cook for 3 minutes then add the green tops and cook for 2 minutes more.
If you don’t have a steamer, bring about 1-inch of salted water to a boil either in a wok, my personal preference, or in a wide sauté pan. Stir-cook the greens in the boiling water until the white stems are just tender, about 3-5 minutes. Lift the bok choy out of the water and quickly drain on paper towels before arranging on a platter.