def time[A](f: => A): A = {
  val t1 = System.currentTimeMillis
  val ret = f
  val t2 = System.currentTimeMillis
  println("%g secs".format((t2 - t1)/1000.))
  ret
}

val lb = collection.mutable.ListBuffer.tabulate[Int](1000*1000)(identity)

time(for (j <- 0 until 10000) { lb.length })
time(for (j <- 0 until 10000) { lb.size })

scala> time(for (j <- 0 until 1000) { lb.length })
0.00000 secs

scala> time(for (j <- 0 until 1000) { lb.size })
5.48300 secs