If you are measuring rotor-tip to rotor-tip, then that sounds right. I believe the fuselage itself is around 11-12 inches long. Don't have the A/C in front of me at the moment (currently at work), but it may be a bit closer to 20 inches.
Not sure what guage styrene they used for the fuselage halves, but i can't imagine something that big being anything less than .020 or .025. So strength may not be an issue. If all else fails, you can contact them just to be sure.