If I was to theorise...The Federation in 200 years, went from being a handful of planets to spanning across 8,000 lightyears. If you take that level of expansion and have him remain consistent, the Federation by the 31st century would be nearly 70,000 lightyears in size. That's debatable however as the Federation would probably spread to nearby galaxies. Not every race in the Milky Way Galaxy is going to give up independence and join the Federation, no matter how beneficial it might be.
We know that by the 29th century, the Federation is temporal based with time travel seemingly a routine part of civilization. What this actually extends to is unknown, but it could be part of a bigger picture involving the Temporal Cold War. Daniels claims to be from the 31st century, I say "claims" because it was never confirmed who he actually was/is and Archer only had his word to go on.
I like the idea of the Federation collapsing at some point in the near Trek future. It would make for an interesting era. Say 50% of the Federation became independent once again during a war or perhaps a crisis, it would be very interesting to see how the balance of power in the galaxy would shift. I think fanon and even licensed novels can go a bit far sometimes. I think it's ridiculous for people to consider ideas like the Borg joining the Federation or even the Klingons becoming full members, therefore giving up their cultural standards because they aren't legal in the Federation.