It took several different tries and a few pounds of ABS plastic, but I believe I finally settled on a good modular shape that can be scaled quite nicely. As the files are they will produce 32X24.5mm cubes. Scaling down to 50% or even up to 200% has produced good results.
How To Print:
Printed the pieces on a RepRap at .5mm height. 20% infill. Single shell.
I would advice to just print the "squareBrick_Pallet.stl" to get a single of each of the 8 different parts to get a feel for how the pieces work. Then once you can get a ballpark of how many of each you want you can use the individual part STLs.
How To Build:
The thing to keep in mind is that all the pieces in the set are based around the single two main parts (squareBrick_Half and squareBrick_Whole). And each piece has both a whole(24mm high) and a half(12mm high) version. The key to the bricks strength is when you alter the height from half to whole parts and build the wall up from that.
When building, start with just those two pieces and build out the shape of the object you want. Then using the other six pieces you can go back and cap and cover the exposed joints.
I've included a CGI image to show ideas and to get you started on what goes where. Each color is a different piece, for 7 total (whole_cap_longer isn't shown in the picture, making a total of 8 different pieces in the set).
I'd like to go back and alter the "cap_longer" pieces to fit inside corners.
Make some double length pieces. I wanted to keep the different piece count down to start with though.
Compatibility pieces. To connect to Legos and other building sets.