I've prepped an image for a billboard - and that was a REAL learning experience! I thought I was going to have to make enormous files for them - nope...

The printer told me that highest that would possibly be printed by a billboard company would be 36dpi - that's only for SMALL boards like 10x12ft.

The one I did was going to be printed at 12dpi as a 10x20ft banner - that was only going to end up being 1440x2880! What helped with my prepping was that only about 2/3rds of the image was going to be photographic anyway - the rest was solid colors and vector graphics - I just set up the file in the exact format it was going to be printed and filled it accordingly.

When they print boards for the sides of buildings, or extra-large signage you drive by on highways, they typically print 9dpi! Not kidding!

It doesn't have to be huge - it really doesn't! Ask the printer what is going to be the output - and they don't play the games that publishers do (demand it be 300dpi for their output size when they only print 150 lines per inch!). They tell you exactly what's going to be printed because THEY don't want to deal with uselessly enormous file sizes.

Good luck-
-Lew ;-)