I have had this discussion before with the TFL folks as yes, some of the scenario books don't mention a size.
-If you see a grid, then assume these are 12-inch by 12-inch.
-with no grid, I like to use a ruler to get a sense of the relative length of the "long-side" and the "short-side"
For example, if you measure on the paper printout from the scenario, that one side is 4 inches long and the other side is 6 inches long, then presto!? there is your 4 foot by 6 foot table.
I would not sweat it too much. The maps are very impressionistic so do what works for you with the gaming tables/plywood/foam pieces you have.
