I don't know if this should be considered a derivative of http://www.thingiverse.com/thing:591 or not. It was inspired by it but I started from scratch because I wanted to do it in openscad.
You can adjust the size and tweak the dimensions of the various parts of the lego in the openscad file.
There are probably bugs, it's a work in progress.
A real lego snaps into the bottom pretty well, not as well on top.
The script defines a module called block(width,length,height,axle_hole,reinforcement)
The width and length defines how long and wide the block is (surprise!). They are defined as the number of knobs in that direction, not the actual length in mm.
The height is a multiple of the standard height of a brick.
The axle_hole variable adds holes to the top of the brick to fit standard lego axles through. They may only fit partway through unless your printer can handle really thin walls.
The reinforcement variable adds ribs across the brick to reinforce it (which also makes it print nicer because it gives the bot somewhere to traverse between the posts on the underside of the brick without leaving strings everywhere)