I am growing zucchinis in a raised bed and am wondering the timing for harvesting the zucchini. In prior years, I had zucchinis of many sizes and shapes (will try to dig up the photo and post). However, I am trying to determine this year the timing for harvesting zucchini. I know the guideline is to use what is on the seed packet but the zucchini is not a one time harvest. It keeps on giving over a period of 1-2 months. (Ignore the cucumber in the 2nd picture.)
If I understand you right, you just want to know what size to harvest your zucchini at.
Most people harvest them when they're small (maybe four inches long, give or take a few inches). They think they taste better then than when they're huge. It's a matter of preference, though.
I, being rare, personally prefer them large (about half size to probably just before they reach full size or about the time they do). One of my reasons for this is that they lack that sticky substance that immature squash have. They taste more substantial. When you slice them, they're the right size for a sandwich or hamburger. However, if they get too old, they can taste bad and have thick skin. Some of them taste great old, though. I'm not sure if that's the variety or the soil or both that makes the difference.
Smaller zucchini are most likely the best choice for salad. Bigger ones are great for zucchini bread.
Your question is quite unclear. Generally you can pick Zucchini whenever you want - although the ideal is supposedly about 6 inches (15cms). As they get bigger they apparently loose their flavour and texture.
It is unclear why you think the multiple harvests are important - but you will find that picking them smaller encourages growth of more plants (but you will probably also find that by weight, fewer larger zucchinis will provide a larger yield.