Is the pup truly a pup? If so, get one that's small, but s/he can grow into (an intermediary bed of sorts). IMHO, a bed size depends on how they like to sleep and also, how big they're going to get. Honestly, if I bought a bed based on Koji's size, I would've been wrong.
See attached photo.
That's Koji, in Miko's bed. Koji is about 17.5" tall, Miko about 15.5. Miko likes to sleep in a ball up against things (hence the bolsters). Koji likes to sleep against things, on his back, fully laid out or on his side, fully laid out. Koji would need a very large bed, maybe 30", Miko is okay with 24-26" (that's the sleep area, not the actual size of the bed - again, because Miko likes bolsters).
That's why it depends on how the dog sleeps. Miko wouldn't ever use that kong bed that I wish I had so Koji wouldn't eat it, because he wants bolsters. Those donut beds, ah, Miko likes those. If I was to buy him one when he was a pup, I would've bought a small one that fit him, but was big enough he could grow into it a bit. Once he was too big for it, I'd buy him one for his full size.
Hope that made sense. Good luck. If unsure, buy cheap first. Especially if the pup is prone to chewing.
ETA: Found the dimensions for Miko's "good" bed. It was a donut bed and these were the dimensions for it: interior space 26"x17"x6"
btw, I never knew they MADE Kong beds. Too cool. Would've been perfect for my chewing boy Koji.