Here’s an old thread, not sure how helpful it is as some of the posts area no longer visible.
Hi Alex,
There is absolutely no way you need 72dpi for something that big. You’d easily get away with 5dpi. Have a look at a billboard up close one day.
As a rule of thumb, the minimum viewing distance, which allows the whole image to be viewed in its entirety, can be calculated by multiplying the length of the diagonal of the item by 1.5.
Then use this handy table to work out the required dpi for continuous tone (ie. photographic) images
[viewing distance-ppi]
If you have vector art or tex…