Ballisa scale fine. I have one about that size. It shoots arrows 100 yards roughly. I would recommend starting with an onager. They are really much easier to make, and better at throwing projectile than ballista (which are a pain to make throw stuff other than arrows. I never got mine to work with non arrow projectiles). Onagers are much easier than ballista anyway, and work on the same torsion principal. If you get one to work, you might then progress to a ballista, but learning about torsion first on an onager will save you having to fix every mistake twice (as ballista have 2 bundles). Also, an onager could easily meet your requirements, so if it takes too long, you could just use it. I have an onager much smaller than that which has thrown gold balls over 200 yards (600 feet!). A ballista would probably be more accurate though.
Be sure to check out the torsion section.