Get someone to shoot a 4x5 positive, then have that drum scanned. Expencive? yes, but it gives you decent quality. Now, something to remember is most billboards are UNDER 100 Megabytes saved as TIFF, so just remember viewing distance.
best thing is to save your file and
Image-->Image Size, set to how you want it
View--> Print size and thats how the print will look.
Thats what you need to do in photoshop to see how anything will look.