ScalaCheck.scala

import org.scalacheck._
import org.scalatest._
import org.scalatest.prop.GeneratorDrivenPropertyChecks


class SubsetGenerator(set: String*) {
  protected val subsets = set.toSet.subsets.toSeq

  val arbitrarySubsetGen: Gen[Set[String]] = for {
    subset <- Gen.oneOf(subsets)
  } yield subset
  implicit val arbitrarySubset: Arbitrary[Set[String]] = Arbitrary(arbitrarySubsetGen)
}

class SubsetSpecification extends FeatureSpec with GivenWhenThen with ShouldMatchers with
GeneratorDrivenPropertyChecks {

  feature("Subsets") {

    scenario("The size of a subset should be between 0 and 3") {
      val sg = new SubsetGenerator("One", "Two", "Three")
      import sg._

      forAll("subset") { (subset: Set[String]) =>
        whenever(subset.size < 3) {
          Given("the subset " + subset)

          And("the size should be < 3")
          subset.size should be < 3
        }
      }
    }
  }
}

Instead of writing your own generator, you can use ScalaCheck's Gen.someOf that generates a subset of a list of elements:

...
  feature("Subsets") {

    scenario("The size of a subset should be between 0 and 3") {
      val sg = Gen.someOf("One", "Two", "Three")

      forAll(sg) { (subset: Seq[String]) =>
        whenever(subset.size < 3) {
          Given("the subset " + subset)

          And("the size should be < 3")
          subset.size should be < 3
        }
      }
    }
  }