Update: I am leaving this here for historical reasons mostly. In the meantime, I have come up with better boomerang designs. Look at my other things if you're interested in printing a boomerang.
A simple tri-blade boomerang.
Look anywhere on the web for throwing instructions, e.g.: http://www.rediboom.com - this is an actual returning boomerang!
I printed it in ABS; size should fit into a 20x20 cm bounding box, thickness should be 3.5 mm. Making it a little larger won't hurt, but the 3D printing company I used only has a 20x20 printing area.