What size bed for English Bulldog?

Hi, I want to purchase a bed for my English Bulldog, but am unsure what size to buy. She is a 5 month old female 30lbs currently. I want one that will last and that she can grow to her maximum size, but still fit comfortably. She likes to sprawl out when she sleeps either on her side or back. She also likes to lay her head on something so I am thinking of getting one with a bolster. Any recommendations? For size reference her pic is below. Thanks!

http://imgur.com/JQdP4M2