In total it was 374 textures, GUI elements and icons, mostly at 16x16 pixels. It was very interesting working at such a small scale, you really have to consider each pixel. Subtle colour changes and shading are really key.
I've taken another look at Portal 2's Perpetual testing initiative puzzle creator. It's a really nice simple system that allow you to quickly prototype and test puzzles. Thus far I've put 3 together, enjoy :D