That's my photo on a billboard. Just past the Thurbers Avenue Exit, I-95 northbound.
The original was taken with a Panasonic Lumix DMC-LX3, which is a 10.1 mp point-and-shoot. Billboards are typically 14 feet tall by 48 feet wide. This is a bit larger than half the width of the billboard, so maybe 26 feet wide?
Think about that next time you wonder "how large can I print this".