When to harvest what

When I first started growing, I harvested things when they looked like the ones in the supermarket

The right moment seems to come with experience really, it's a bit trial-and-error to begin with.

Onions, you can harvest them individually as soon as they seem big enough to use. If you want to store them for use later, leave them in the ground until the leaves have fallen over and turned brown, then hang them up indoors to dry for a bit longer.

Beetroot, you can harvest at any size from golf ball to tennis ball sized. They're more tender when they're smaller though.

Beans, if you want to eat the pods, harvest them when they look big enough to eat. Some types don't get seeds in them until they're a bit big and tough. Unless you want to eat the actual beans, rather than the pods. In which case, wait until you can feel them in the pods. If you want to save the beans to dry and use later, then you wait until the pods go dry and papery.

Cucumbers, usually if they've turned yellow then they've gone past their best. The test is usually once the sides of the cuc are parallel, but that doesn't allow for home-grown weirdnesses in shape. Better to pick them too small I'd say, than leaving them too long.

That's very useful info. I hadn't much of a clue, but now I have.