1
Set up the file so that the dimensions in PIXELS is 1620x1620.
2
It does not matter what dpi you use as the sign only cares about the pixels, not the dpi resolution. So, you may want to use 72 dpi so that the file size is easier to work with.
3
Use the highest possible original resolution for all graphics and photos which are bit-mapped. Try not to use web type low res images, instead go for print quality images. Do not reduce the res in your software leave them at full resolution - so you should end up with a huge file to send. (If you end up with a small lower res file it will work, but image quality will be reduced).