Catrin I used the smaller one on my 47 cm Trek 5200, although I am not really sure if DH switched out the carbon seat post, despite all of the warnings. I had that bike when I first started commuting and you would not believe how much stuff one can get in there. I once rolled up a stretchy t shirt dress, underwear, and make up in there. DS now has it on his steel Serotta and it works great for him.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport