I did a whipper conversion of my small frame. That's a modern take on the ancient weapon that gives increased performance.
For a size comparison those are tennis balls in the photo.

Also have a short vid on You Tube.

http://www.youtube.com/watch?v=hR5zFYBsd5A

Bill Skinner wrote on Jan 6^th, 2011 at 10:36am:

Ya know, I'm not quite sure. An indirect guesstimate tells me 110 to 115 feet. I was throwing from the back yard toward the house and cleared the ridge which is about 17 feet high and 90-95 feet from the firing point. Launch angle was steep and there's another tuning point that needs addressing so there's a bit to be gained, maybe a big bit.

Bill, trebs are addictive if you're susceptible so beware.

I use frame plans that are in my head, basically my own design, scaled up or down. All the boards are cut according to a formula based on the desired frame height and board width. Nothing really published. There are a couple of fairly simple sets of plans on Ingenium, one small the other bigger that are OK.

http://www.tasigh.org/ingenium/medium.html

The thing to remember is that the frame is just a bracket that holds the working parts in alignment. It's the arm, counterweight, and sling that do the hurling. 90% of all problems come from doing these wrong. (85% of all statistics are made up, too.)