Summer squash can grow quite large (home gardeners often discover baseball-bat-sized zucchini hidden under the plant’s large leaves), but these overgrown specimens have coarse, stringy flesh and large seeds. Summer squash taste best when small- to medium-sized—not more than 7 inches long (pattypan squash should be no more than 4 inches across).
Choose squash that are firm and fairly heavy for their size; otherwise, they may be dry and cottony inside. Farmers’ markets and greengrocers sometimes offer baby summer squash, just 1 to 2 inches long. Baby squash are particularly tender and sweet.
The skin of summer squash is thin and fragile, delicate enough to puncture with a fingernail. Unfortunately, some shoppers do just this—they prick the skin to test for tenderness, leaving the squash susceptible to decay. Look for squash with sound, glossy exteriors. Avoid those with skins showing nicks, pits, bruises, or soft spots.
The squash should be plump (not shriveled), the stem ends fresh and green. The color should be uniform and bright.