Well! Thanks to magical powers of Photoshop, I think I found out the roster size! ... Maybe.
So! Here's the roster of the demo:
It has a total of 20 characters + a random slot. Pretty solid for a demo.
Now, lets add some slots to make that a perfect rectangle.
This gives us a whooping 23 slots. There's still some space to the sides, though. Lets cover them!
... Could only add 3 more. So here's a 27 big roster. Seeing that we have 34 characters revealed, there's clearly something wrong here. Let's do the same as Brawl and shrink those slots!
Now we're at 48 slots... huh. It seems whoever did that estimation was somewhat right. But, assuming the Brawl crew comes back with the lowest amount of cuts possible (two, Ivysaur and Squirtle) at 37, plus the 9 newcomers shown, there'd be only a single space left in the roster (Mewtwo?)... which, unless it means there have been a boatload of cuts, means that the roster is pretty much underwhelming. So many popular characters are being left out.
However, let's get a little crazy here. So, we had 64's roster, which had 2 rows; then Melee, with 3; and then Brawl, with 4. Let's say this game does add one extra row to it's roster. That'd bring the roster size to...
... an outstanding 60 slots! That is quite a large amount of smashers, would't you say? Though, 59 is a bit of a weird number. Plus, this roster isn't as wide as the demo roster. Let's expand it a little bit more one more time.
Here's the best possible roster. 65 slots - 64 players and a random slot! Not only would this size be the ultimate fanservice Sakurai could deliver, but the amount of characters is 64; a perfect throwback to the original game!
Now, I don't mean to hype you up, nor do I want to dissapoint you; I just did this for fun. Though it gives me a really cool idea of what the roster would be like.
Brainstorming here.