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.