Either base will work, but which base you use is determined by your local building codes. Please consult your local code authorities to determine if a concrete slab is required. If you are able to use a pea gravel base, it should be a minimum of three inches thick. The following instructions come from our installation guide (http://goo.gl/lLxga
"Dig a rectangular area approximately five inches deep and about six inches longer and wider than the footprint of the generator. Cover with polyurethane film and fill with pea gravel or crushed stone. Compact and level the stone."
If a slab is required, it should exceed the length and width of the generator by a minimum of six (6) inches.
The dimensions of the 8kW Guardian Series are 48.3" long x 25.3" wide.