size

// Create a deque with an initial capacity value of `9`
val deque = Deque(9)        // deque == {0, 0, 0, 0, 0, 0, 0, 0, 0}

// Deque size will be 0 as no elements have been inserted.
assert(deque.size == 0)

// Add some elements to the deque
deque += [1, 2, 3, 4, 5, 6] // deque == {1, 2, 3, 4, 5, 6, 0, 0, 0}

// Deque size will now be 6 as we appended 6 elements to the empty deque.
assert(deque.size == 6)