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file size means nothing, viewing distance is everything.
Where will this be seen? how far away are the viewers? ive shot billboards with 4mp cameras. ive also shot a 80x120 image (upressed with Genuine Fractals) for a shopping mall w my d700.
for a nose-to-print viewing distance, a general rule of thumb is 250-300 dpi. At that resolution, it's hard for the human eye to discern the pixelation. (also why Apple calls their new high density displays "retina" displays)
However, if the print is for a large sign/billboard, or in a high school hallway, and the minimum viewing distance is farther than a few feet away, you can start to slack the dpi requirements without sacrificing visible resolution. The same reason why a pixelated image can look sharper after being downsized.