What size diapers do I get?

I am throwing a baby shower for my best friend in August and trying to get a head start on some things... I want to make a diaper cake but I want her to be able to actually use the diapers on it. Her husband is almost seven feet tall and she is about 5 foot 9 and both are kind of a bigger build so I am assuming that their baby is going to be 8 to 10 pounds when born. I am also assuming that newborn diapers will probably not fit but I don't really know because I am pregnant with my first baby so I have no experience in diaper buying yet LOL!