Thanks for writing. You can harvest zucchini as small as you want --- as a general rule, the smaller the fruits are, the better the flavor they have.
As far as how much to water... The general guideline is that most vegetables do best if they get about an inch of water a week. It doesn't matter how --- if it's from the rain or you. One easy way to measure is to put a small inch-deep dish in your garden. Water until it fills up.
I'd avoid watering at night. The best time to do it is in the early morning. Watering after 4.00 p.m. or so can encourage diseases in your garden.
---Justin, Senior Garden Editor, BHG.com "