Chatting with a local mechanic, a shop will usually chop the seatpost ahead of sale, so you mustn't have to bother with this far too much but if you are doing must, You should not worry since it can be achieved! Just for reference this was for a size 51cm https://cervelo-bikes-for-sale70533.therainblog.com/26217917/helping-the-others-realize-the-advantages-of-kids-bicycle