They are talking about the gauge of wire you are using to wire the flexwatt. The cord you buy will have that information on it, the gauge of wire it is.
The description of the flexwatt itself tells you the wattage it carries. For example, the 4" flexwatt says it carries 8 watts per foot, and can be used at a maximum of 60 feet.
Any heavy duty extension cord should easily handle 40 feet of flexwatt. Maybe even the entire 60. I have a simple white cord I bought in the lighting section of Wal-Mart for around $4 wired with 25 feet with no worries.
Gale