Bubble nests aren't actually a sign of happiness, just breeding readiness. There have been many cases of very sick Bettas making bubble nests in tiny little cups. When I was much younger, my best friend and her sister had one in a tiny, tiny, tiny little bowl, not much bigger than a 12 oz mug with very dirty water that they rarely changed and the entire thing was filled with bubbles and he continued to make tons of bubbles everyday until he got so sick he couldn't move from the bottom and he drowned. (I was very young and didn't have any fish yet, and I certainly didn't know any better than, and obviously, neither did they)
As for your other question, I have seen pictures of bubble nests expanding across most or all of the tank, so they can get quite big.