Hi Alan,
Clif is correct, the scenarios ("Adventures") say they are written for a 6'x4' table.
The scenarios use between 4 to 8 "Large" area terrain pieces. "Large" is defined as: "not able to be contained within a square of 15cm by 15cm, and the two points of their circumference that are the farthest away [from each other] should not be separated by more than 30cm." Page 59 (the part in the brackets is my interpretation).
I think that means, small enough to fit inside a square foot, but too big to fit inside a 6" x 6" square (taking some liberties with metric to Imperial conversions).
Area terrain should be your typical flat, oval-ish shape, with movable trees, boulders, tall grass, etc, for visual effect.
"Small" area terrain occurs in only a single scenario (just one piece), and is defined as "must be fully enclosed within a square of 15cm by 15cm." Page 59.
The following items show up in one scenario each (not the same scenario!):
4 tents
3 huts
Large Tree
Large Stone.
The other thing you want lots of is "blocking terrain," which is dense bases of vegetation or rocks, of no defined size, but the examples in the book look like no more that 2" or 3" on a side. These block line of site, and cannot be entered by troops. The scenarios just say to scatter this stuff around, but don't state numbers or exact placement.