I don't have that off the top of my head, but you might consider a look at
billboardsofthepast.com
which has some nifty examples that come with measurements!
It looks like something on the order of 10 x 20 feet is the ballpark.
"Transactual" has been selling billboards on eBay in Z, N and HO sizes (disclaimer: happy customer but no other connection) and I think he has them sized to 28 scale feet wide. Of course you could use the smaller scales' signs as well.
