I'm really not even a baseball fan but I love this place! I was given tickets from a friend for a Marlins game last season so I was thankful that I got to see a game here in it's inaugural season. 318 is where I sat, obviously, row 10, seats 6 and 7. We didn't sit there the entire game, we moved down a few rows. You can pretty much sit anywhere you want in the 300s, it's a ghost town up there which is why they're closing the upper deck for weeknight games like they used to at Sun Life Stadium. That's pitiful when you consider the Marlins play in one of Major League Baseball's smallest venues.
Anyway, there was plenty of width between each seat. Plenty of arm space too. The seats are a unique shade of blue and the outfield wall is unique shade of green. On tv, this place is ugly but it will grow on you if you visit.
We were able to access these seats via a ramp but we later found out there was also an escalator as well that could have saved us some time and energy. We'll remember that next time.

So even if you do not like baseball, Marlins Park is definitely worth the visit at least once. There was plenty of parking, plenty of good eats, and plenty of things to do (party at Clevelander or visit the bobble head museum on the main level).