I think an 8 x 16 meter Raster file would be scary
But then I've never done a billboard.

As for the rez...contact the person that will be printing the billboard and ask them for thier specifications.

Your really gonna do this totally in Photoshop?

Ask the printer how they recommend. Also remember that a billboard is very high up and a distance away (for the most part) and you won't see minor imperfections like you would in an advertisement.

I would recommend doing most of the work in a vector based program, but you can use Photoshop to a certain degree. But like I said, contact the printer and find out how they would like it.

Definitely contact the printer. They usually have a scale/dpi ratio for you to use.
Don't be scared if your dpi is below 20 at final size. Actually, don't be surprised if the dpi is ipd. This is almost as big as a building wrap where the "pixel" dots can be 2"x2".
You may have to do something this size in sections with bleed for welding. I know in Europe there are mega-wide printers that go to 8 meters but very darn few in the US.
Hope you have a good processor and lots of ram.

You don't need as high resolution for billboard as you do for smaller print (ie, you may only need 100dpi or so), but you must check with your printer for the exact specs.

I really wouldn't ever do an 8x16 meter billboard in raster format, myself. It can be done, but it's definitely not suggested. How good are you with Illustrator? If you need a photographic image, I would create a photo-realistic illustration in Illy, even though it will be labour intensive. If they can pay for the billboard, they can pay for the work that goes into designing it.

Computer will asplode.

Photoshop will first. Your computer is safe.
At 15dpi as a .psb that's a 16gig file. Good luck with that. I don't know many people who would print a .psb let alone 16gigs of it. But I don't talk to building wrap people if I can help it. I just pawn off those printing nightmares.

Which program to use?

I am designing a billboard that will be 14 x 48 ft. I have done a smaller version to scale in Illustrator, but it will not let me create a 1:1 file because the dimensions are too large. Any suggestions?

I am an In-house designer for a large format digital print company. Every print company has different specs as to what they want. We generally want things at 1/4 scale at 300dpi unless it is smaller than 12 feet. In that case we want the file at a 1:1 scale at 400 - 500 dpi (depending on just how small it is). PLEASE FOR THE LOVE OF GOD, ALWAYS CONTACT YOUR PRINTER FIRST IF YOU DO NOT KNOW WITHOUT A SHADOW OF A DOUBT, THE SPECS ON HOW THEY WANT THEIR FILES!! I say this because I deal with the headache everyday of ad agencies that sub contract their design work to us and provide us with crap... and the outdoor sign companies that send stuff for us to print that is absolute crap and I have to rebuild it for prepress!

Edit: ...and yes, I have done building wraps and also huge mesh vinyl boards that are stretched between two buildings. I usually will do these in sections or quadrants in illustrator. Those files are huge and get printed separate and welded together. It is a major pancaking headache!

Printing it in sections sounds great! I think I will go pick up a printer at wal-mart and begin printing my 100'x200' self portrait for every major highway! I will have to calculate how many 8.5x11 sheets of paper that will be....!

100/8.5=? carry the 6...measure the mm....subtract my common sense....and presto!!!! ummmmmmm never mind!

Exodus, 1:1 at 500dpi? Really? I don't want more than 200dpi at 1:1. Unless it's an 'art' print.
See, we are all different.
I second his comment. CALL THE PRINTER for the exact specs.
Depending on the output device I'd want 1:10 at 350-2000dpi scale or 1:12 scale at 420-2400dpi in Quark. And 1/4 scale or better in the programs with real artboards.

PD, 500dpi @ 1:1 is for stuff less than 6'. 400dpi for stuff 6.5 -> 12'. Over 12' I want .25 scale @ 300dpi.

Edit: Yea, we get a lot of art murals and such that has to be really high-res.

Thanks for the suggestions.

The resolution isn't the issue here, because it is primarily vector. My question pertained to the method of creating the file. I went ahead and made the new Illustrator file in 1:4 scale and that seems like it should work. Please let me know if I am missing something here...

The billboard company is Lamar, and I have no idea who the printer is, as my client is in California and I am in Atlanta.

Thanks again.

Prolly us. We're in Chattanooga, TN.