可以使用以下方法修复第一种方法：

class SubOfA() extends A {

    override def isLessThan(that: A): Boolean = that match {
        case that : subOfA =>  this.size < that.size
        case _ => throw new UnsupportedOperationException("Wrong comparison") //or whatever behaviour deemed suitabe here
    }

}

你可以用 F-Bounded 多态性（java上的解决方案是什么），或者 Typeclasses 多态性（这将是haskell的解决方案）。
我个人的偏好是使用typeclass，因为它更具可扩展性、可维护性和类型安全性——下面是robnorris所作的更客观的比较。

f-有界。

trait Comparable[A <: Comparable[A]] { this: A =>
  def isLessThan(that: A): Boolean
}

class SubOfComparable extends Comparable[SubOfComparable] {
  val size: Int = ???
  override final def isLessThan(that: SubOfComparable): Boolean =
    this.size < that.size
}

类型类。

trait Comparator[T] {
  def isLessThan(a: T, b: T): Boolean
}

object syntax {
  object comparator {
    implicit final class ComparatorOps[T](val self: T) extends AnyVal {
      final def < (that: T)(implicit C: Comparator[T]): Boolean =
        C.isLessThan(self, that)
    }
  }
}

class Sub {
  final val size: Int = ???
}

object Sub {
  implicit val SubComparator: Comparator[Sub] = new Comparator[Sub] {
    override final def isLessThan(a: Sub, b: Sub): Boolean =
      a.size < b.size
  }
}

import syntax.comparator._
val a = new Sub(...)
val b = new Sub(...)
a < b