Regina,
How big is big? 4 foot, 8 foot? What will it be printed on? Canvas? In Photoshop, try this. Go to "Image Size" and the box will dropdown. Will show the current size. First duplicate the original, so you're not messing with that image/file. Then in "image Size", next to "width" and right under that "Height", you'll see to the right, usually some numbers and usually "pixels", click on the drop-down arrow, and select "Percent", then change the 100% to 110% and hit OKAY. Then do this over again. You'll see the "dimensions" change and the pixel size. Keep doing this until you reach where you think you need to be and then stop and take a look.
I've sold 4 foot x 6 foot prints here when I first joined, from pretty small files, so this might be an easy fix, depends on WHO will print this? Your client or FAA? If YOUR CLIENT, then need to have your client find out from THEIR PRINTER what size file THEY NEED and if they themselves can upgrade to a larger file, which most pro labs can do.....
Rich