I built these to serve as scale models for a project I'm working on. I wanted to absolutely confirm that they'd fit together as intended before I spent a bunch of money on materials and wasted a bunch of time.
In the STL files, each component square is about 1 3/8-inches across. As long as you keep them at the same scale, they'll join together as you'd expect.
I don't expect this model to change much. It was created in 123D Design. If you'd like the original source files, leave me a comment and I'll post them.
I printed them successfully in PLA on a Replicator 1 and Replicator 2 with 2 shells and infill at 2%. You may need to put pads on the corners of the S and the T if your machine has adhesion issues.