Given to family for DIY Christmas Presents. 

Roughly palm sized. Requires a 1.5in (about 37mm I guess) drywall nail as a hinge pin and a rubberband. (I guess you could print this part, it just happened to be what I used as a base for the entire design) 

Models include one "allparts" so you can print everything in one go, as well as each part individually.

Either use glue to fit all the parts together after you've run the nail hinge pin through the throw-arm and stopblock parts, or do what I did and use a second rubber band to hold it all together. For me all of the parts fit almost snug but not quite. Probably will print fine without supports on most printers. The only concern would be the male connection pegs of which there are 4.

There are certainly better and more elegant catapults out there, but this is mine and basically my first 3D model ever. Made in Blender