No because if you multiply each dimension by 10, you end up with the
volume (and hence the mass) going up by a factor of 1000


240 * 20 * 12 = 57600

2400 * 200 * 120 = 57600000

57600000 / 57600 = 1000

But that doesn't tell us if your original model uses that particular x-
section just because it was readily available, or possibly because it
"looks right" - rather than it be designed correctly for the stresses
on it.

I doubt the medieval designers of the real thing could calculate the
stresses either though (and it's a far from simple problem), so maybe
just go on experience and instinct.

Masses of online resources:

http://en.wikipedia.org/wiki/Trebuchet

However simply throwing bricks at non-paying customers would be a lot
less hassle.