Data Science Tools Using Scala
Re: size of data

The short answer

Scala works for both small and large data, but its creation and development is motivated by needing something scalable.  Scala is an acronym for “Scalable Language”.

The long answer

Scala is a functional programming language that runs on the jvm.  The 'functional' part of this is a fundamental difference in the language that makes you think differently about programming.  If you like that way of thinking, it lets you quickly work with small data.  Whether you like it or not, functional languages are fundamentally easier to massively scale.  The jvm piece is also important because the jvm is basically everywhere and, thus, Scala code can run basically everywhere.  (Note there are plenty of other languages written on the jvm and plenty of other functional programming languages, and languages beyond Scala do appear in both lists.)

This talk give a good overview of the motivation behind Scala.

Re: other tools that have good Scala support:

As you mentioned, Spark (distributable batch processing better at iteratative algorithms than its counterpart) is a big one.  With Spark comes its libraries Mllib for machine learning and GraphX for graphs.  As mentioned by  Erik Allik and Tris Nefzger, Akka and Factorie exist.  There is also Play.

Generally,  I can't tell if there is a specific use case you're digging for (if so, make that a part of your question), or just want a survey of big data tools and happen to know Scala a bit and want to start there.