To make it bigger I would consider changing the direction of the top, that is the football field would be cross-wise instead of lengthwise. Then you would add in more borders top/bottom, maybe one along the side.
I might even trim down the current outer border just a tad, then I'd probably put in some sort of white/light and probably pieced narrow border (2-4"), a 6" or so and then finally about 8-10". You can even go off the color scheme for those wide edges, maybe an all-over brown football print?
And of course there are the constraints of what does the store have and how much time to you wish to devote.
Good luck, it's a fun piece for a football fan and just as it is is a good size for watching tv or going to a local game.