Size of seed and size of seed cavity are a function of what varieties you grew.
Generally speaking, cukes start out a very pale green, then darken. They can be harvested anytime they are in that dark stage (I'm assuming green cucumbers, of course, which are the most common).
Once past their prime, the cukes will start turning yellow. Once that happens, they aren't the best tasting; often bitter. But this is the point where the seed begins to ripen.
If you want to save seed, let the cuke go fully yellow and start to turn soft. Pick it, and put it in a safe place, out of direct sunlight, for another 21 days. Then split it open, remove the seed mass, and fement it as you would tomatoes.
When I pick my cucumbers depends on what I want to do with them. I only grow a pickling cucumber, and find that they are good as slicers too. If I'm going to pickle them, I like to pick them when they are small, between one and three inches long, (they fit in the jars better and are crisper). If I am going to just eat them, I let them get bigger. Generally, I only plant cucumbers to pickle, since I'm the only one in the family who likes cucumbers. I try to pick them smaller to pickle, but if I missed one or two the picking before and they got bigger, I eat them. The only bad cucumbers I've ever tried to eat were the ones that start to turn yellow like Brook said. Then they are bitter and nasty.