Portland is the largest Metro Area without a team, at 2,064,336
That's larger than Cincinnati, KC, and Milwaukee. Of course, fan bases and TV markets extend beyond the metro areas, so Milwaukee can draw from almost all of Wisconsin, and Portland would likely attract fans from throughout Oregon.
The next largest non-MLB big city/metro areas (not counting weird conglomerates like Riverside-San Bernadino-Ontario) are:
Sacramento 2,016,702
Orlando 1,861,707
San Antonio 1,854,050
Columbus 1,693,906
Las Vegas 1,650,671
Indianapolis 1,644,250
Charlotte 1,474,734
Austin 1,412,271
Nashville 1,395,879
New Orleans 1,319,589
Buffalo is even further down the list. OKC is below that. Again, you get to draw from the entire state if you're in OKC, but just because a town is big enough to support a basketball team (see Salt Lake City) doesn't mean it's nearly large enough to support a baseball team. It's twice as many games, and quite a few more seats to fill, and the star system isn't nearly as powerful a draw if your team suck (people will pay to see Shaq month in advance ... or LeBron, or Kobe, or Iverson ... I can't say the same about MLBers).
So I guess logic tells us to move the Marlins to Portland! Interestingly, this begs a significant question of realignment, which has to happen at some point because it's untenable, in my opinion, to have six teams in the NL Central and only 4 in the AL West -- huge advantage for those 4 teams, who go into the season with a much better chance of making the playoffs than any of the Central teams. Likely any realignment is going to include the ADDITION of two more teams, so MLB is going to have to figure out how to make that financially feasible. Meaning OKC, Charlotte, and even Miami still have to be considered in the running.