HELP: Exporting Files With Large Dimensions

So I've sat here saving files for an hour or so because there seems to be constant exporting issues that I cant seem to resolve (doesn't help that the actual export can take minutes at a time). I have a 12ft. x 12ft. banner I need to export for printing. I've run into 2 issues. 

1. Saving a PNG. When saving a PNG the exported file will take the bottom part of the image repeat it at the top (cutting the top of the graphic off) and it also flips upside down.

2. Saving JPEG. Because of the PNG issue I tried to settle with a JPEG. The file seems fine but when I put open the file it saves at 8.333 ft and not 12 ft. So it reduces the dimensions when saved. I see no options in the exporting dialog box that would reduce the size of it but maybe I'm missing something.

Is this a common issue? Are there anyways around this? What can I do for save a 12ftx12ft file?

Thanks in Advanced!