Dock a tot!

Hi mommies! 
So I'm almost due in 3 weeks. 
Im going to have my baby sleep with me in my bed and everyone has recommended me a (DOCKATOT) but I find it crazy to buy the smaller size because the baby will out grow it so quick and let's be honest it's not cheap...
So I'm thinking of buying the bigger size so it lasts longer...
Ive heard of some mommies doing something like this but I'm wondering if anyone did that specifically with the DAT?