I'v only uploaded stl files of ukulele. because my printer cannot print any bigger than that. but I made it parametric as many things as possible, so It could easily be scalable. You can generate even a full sized guitar by changing some parameters.
If you want to customize it, copy ukulele.scad and change the parameters on it.
I'v succeeded to print with little ukulele and It was good enough. But I'm not sure on bigger ones.
Sorry for lack of documentation.
If you have any trouble, leave a comment.
I used syvwlch's library (http://www.thingiverse.com/thing:8821) to generate worm gear.
I have to say that this project is failed. I had printed it with standard PLA and 60% of infill. 1 year later, it's neck is bent(strings are floating almost a inch at the center), and I have no way to fix it. Furthermore, tuning gears are all broken. See the last updated pictures.
You can try stiffer materials, but I don't think current dimensions are feasible for FDM printers. I'm thinking about my next attempt to make a novel method to tune strings, and use aluminum extrusion as guitar body, someday..