Look for very bright red strawberries. Unlike tomatoes and bananas, they won't continue to ripen once picked. Small berries can be just as ripe and juicy as larger ones, so rest assured: size doesn't matter. Don't wash berries until you're going to eat / use them. They will mold even if refrigerated once water hits them. I always peak around a box before getting them at the grocery store to make sure no mold has started. And use within a few days.