From the linked article: "Spark weighs in at 8k gzipped and minified, including all of its dependencies, and it's easily separable from the rest of Meteor."
I just looked at the Spark wiki page[1], and Sizzle is a dependency (although they say it can be removed if you're willing to ignore IE 7). So it is separable, but might not be that small depending on your audience.
[1] https://github.com/meteor/meteor/wiki/Spark