Yup, what she said. I'm lazy though, and once most of the seeds inside are black, I just cut off the head, shake into a bowl, and move on. I have so many onion and chive seeds from the last few years it's insane. And those are just the ones I haven't accidentally dropped on the garden bed or kitchen floor.

Onions will set seed the second year they grow from seed or starts. No idea on the little bulbs/starts as I haven't used those in years because they do worse for me than just lazy direct-sown onion seeds.
The root/onion won't be soggy, but several of the inside rings will be greenish (since that's where the flower stalk is coming from). I'm pretty sure you can still eat it if you want, but always check/smell things before eating.
Onion size depends on the variety grown, the length of your season, and the length of your days. I can grow long-day onions up here nearish the Canadian border, but the same onions would flop down in Texas. I can get medium sized bulbs in a good growing year, and it's mostly for fun or to have a few smaller ones on hand for mid-summer cooking. I still kinda wait and get a big 25-50lb bag for super cheap at one of my u-pick farms or whatever in the fall.
