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You can get seat posts in most diameters up to 350mm long. Find out what diameter your post is (usually marked on the post below the minimum insertion line) and Google "seat post ??.?" (replace the ??.? with your seat post diameter) and also check on eBay. Depending on diameter, you might get one even longer. As noted above, you probably won't be comfortable unless you raise your bars and, after that, you'll probably find the reach too short to be really comfortable. Bottom line, there is reason bikes are sold in a lot of different sizes.