Currently, files with large artboards (larger than 6535 px x 4730 px) can not be exported as a PNG with high resolution. The best is a medium resolution. My current project series requires a specific large artboard size (9075 px wide x 6201 px high exactly) and I would like to be able to export and then print them in high resolution to give my customers the best possible printed product. Illustrator does not currently support this, unfortunately.