The beams were standard length. I'm not sure what standard length is, but I believe it's 8 feet. As for assembling the wood/arm... I suggest gluing them together and clamping them, and then once they're nice and even, drill the holes for the bolts. Make sure to use two rows of bolts, although it's entirely redundant since the wood panels won't slip and slide against each other after the glue is set. And I really do recommend gluing the panels because relying only on bolts *may* create extra stress around the bolts, leading to a reliability/safety issue. For a 14 feet arm, there will be a lot of forces involved, and you MUST calculate everything before-hand. Also, if you may want to taper the arm so that its mass decreases further away from the pivot. This will greatly increase the speed of the arm, as well as make it easier to create a stable platform. As for knowing about this stuff... I'm improvising. I'm drawing most of what I say from my physics background. For the counter weights, how about bricks? Flat cinder blocks may be better since larger surface area per brick would allow for less sliding (more friction), and they can be drilled through for better stacking and less sliding/instability. Plus, they don't have gaps and aren't too porous so they should allow for more efficient use of space. You can have a standard weight set and then addition weights that can be secured on top so to that you can adjust the acceleration of the projectile.