WTF Fun Fact - How long is a kangaroo baby when it is born?

The young kangaroo, or joey, is born alive at a very immature stage when it is only about 2 cm long and weighs less than a gram. Immediately after birth it crawls up the mother's body and enters the pouch. The baby attaches its mouth to one of four teats, which then enlarges to hold the young animal in place.

How long is a kangaroo baby when it is born?