The minisphere engine is available in both x86 (32-bit) and x64 (64-bit) variants, the latter of which is recommended on supported platforms as it typically provides better performance. The engine is tiny and self-contained, requiring only 3 MB for the 64-bit engine and its system assets. As it is coded in C, it is also very portable and has been successfully compiled on both Linux and OS X platforms.
As of this writing (June 30, 2015), there is not yet an official site on the Web for minisphere, but the engine is available for download and discussion on the Spherical forums here. The current official release is minisphere 1.4.5. A "minisphere GDK" version is available as well, which includes a console-enabled build of the engine and Sphere Studio for rapid game development.