According to Saint Sheldon you should have about a hand width of seat post exposed from the frame. "About" being the opperative but if you have 1" of seat post or 12" of seat post showing then you might have a look at your frame size. This is based on conventional frame geometry and not frames with slanting top tubes. As far as your seat rocking, take your bike to someone with the proper measuring tools to measure the inside diamiter of your seat tube, OD of your seat post and aslo the seat rails and clamps to determin whish one needs to be changed out to get the proper fit.