You won't get a squash for every flower, just every female flower. Female flowers are easy to spot, as they have a tiny squash at their base. They are also usually attached directly to the main stem, while male flowers grow upward about 6-12 inches or so on their own stems, open for one morning only, then wilt and fall off exactly as you are describing. Most healthy squash plants produce about 80% male, 20% female flowers. You should still get a squash per plant every other day or so during the hot months when they are really growing, sometimes more. Just make sure to pick them early and often. 8 or 9 inches is about right. Summer squash, especially zucchini, can get truly enormous if allowed to do so, but will become tough and seedy. More importantly, if many are allowed to grow large, the plant will stop producing. It has been my experience that they will also only set 1 or 2 fruit at a time per plant, with any other female blooms present simply failing to develop. So regular, steady harvest ensures regular, steady production.
