When to use which:

Hash

• size and length both call RHASH_SIZE which is O(1)
• count is enumerable.count which is O(n) – use only with block
• best practice: length without block, count with block

Array

• size is an alias for length
• length calls RARRAY_LEN which is O(1)
• count is O(n) – use only with block
• best practice: length without block, count with block

ActiveRecord

• count and size create a COUNT query which is often faster than alternative
• length creates an array and calls length which is usually slower
• best practice: count