I tend to think the usefulness of a (standard) library tend to decrease with the size of its interface. Because with more functions, it is harder to learn, harder to find the function you need, and makes programs less accessible because there are so many ways to do the same job.
My bet would be a minimal standard library, based on empirically most useful functionality. I very much doubt you'd need more than a thousand lines to implement that (not counting a possibly crap-load of optimizations).
In my experience, what matters more than the size of the interface of the standard library, is its organization. For example, I can't really read Clojure code, mainly because I have no idea what the various functions do. On the other hand, Python has a huge standard library, but usually the name of the function, in the context of the module (as in urllib2.urlopen or json.loads) or the name of the method in the context of the class of the object it belongs to (e.g. [1, 3, 2].sort()) is enough to figure out what the functionality is.
Though I wish I didn't have to type `from datetime import datetime` all the time...
My bet would be a minimal standard library, based on empirically most useful functionality. I very much doubt you'd need more than a thousand lines to implement that (not counting a possibly crap-load of optimizations).