I'm new here, so I may be wrong! But I believe the issue is not the original size of the work, but the size and quality of the digital image that you upload. The better and bigger the image (from a digital camera or a scanner) the larger prints that can be made. When you upload the image to FAA, they will automatically tell you how big the prints can go. The prints will be made in the same proportions as your original (so the sizes will not necessarily fit standard frame and mat sizes). I just checked my greeting cards, and for those the images may be cropped in order to stay in a 5x7 format.
I'm sorry I can't give you the best image sizes...I'm not techie enough! But I know there are other discussions about it here.
I think graphite should work if it's a strong image to start with.