Who hoo, I did this same exact thing with MS Paint. What did I conclude? That while you could figure out some interesting proportions and whatnot, you really couldn't determine the EXACT spot the character box ended, and with an error like that, you could EASILY fit another character or two per row which ends up adding 5-10 extra characters...pretty big mistake I'd say.
In any case, it does seem the character boxes are about the same size (proportionally to the big character slots) as in Melee, but it is impossible to tell
exactly where they begin and end and everything will give you a rough idea, and that is really no big deal. It's like smash icon discovery...oh, there will likely be at least one extra series added for Brawl. That's a no brainer. If they add
any new 3rd party, we'll have a new series. Just like this...there are
at least 36 chars, well duh. I could have told you that already. But, IMO, I think that number is far too low.
Compare the following
(do you realize how hard it is to find a melee character selection screen on google image search? try it sometime)