Hey man, this is how I do my adboards.....
File size is 512(width) x 128(height).
I make 2 files for each adboard....
1 for fencing 1 and the 2nd for fencing 2,3,4.
Here's an example of how they're positioned....
fencing 1 at the bottom of the file
fencing 2 3 4 in the middle of...