It's not length of the tube. The inner diameter of your seat tube either needs to be just a bit larger than your chosen seat post, or somewhat larger if you use a shim. Could be a bit of a soda can, could be a variety of production shims from Problem Solvers. I buy cheap 27.2 seat posts for the kid bikes and cut them shorter. I think I used an externally butted bmx seat tube for one frame and a non-butted tube sized for 27.2 posts for the others. Wicks and Aircraft Spruce list inner diameters, and you can find shim sizes online. It's worth looking before buying. I had one very large seat tube for a tandem I made that shims weren't available (that I could find), but I have a lathe and had some suitable scrap to make a shim.