Okay, after about 1 minute of googling, I did find reference that the MTS Centre is now actually small for the NHL arena seating standards.
http://www.macleans.ca/article.jsp?content=20070529_092340_13592
Some blogs have pegged the new minimum for new and relocated teams at 17.5K seats, but nothing that is in an article I would cite.
The NHL has been smart to not have a said figure, however.
The more I think about it though, perhaps 15,003 does work in today's NHL. We'd probably be at the break-even point at best, however. NHL is seemingly in talks with Chipman, so they'd probably make an exception just to spite Balsillie. Though, that would only happen if Balsillie looks like he could actually get the Coyotes.
All that said, is anyone actually naive enough to trust Gary Bettman of all people?! Regardless what the minimum specs is for arenas to host an NHL team, they could change it or enforce in a heartbeat to screw Winnipeg.