Is size the main selling point? 4 years ago or so, I integrated SpiderMonkey into a C++ program with relative ease; it has an easy to understand C API and no dependencies.
It's not tiny though (for an underbelly feeling, I found an old binary Windows build online of about 800kb).
Looking at the current SpiderMonkey docs, things look like they've only improved. Back in the day, I just took SpiderMonkey from the "js" subdirectory of the Firefox repo. They split it off and documented build steps since then.
I'm not saying there's no space for contenders here, but don't underestimate the quality of what's out there already. SpiderMonkey really is remarkably easy to interface with.