The 19', 21' is the frame size. The Frame size is measured in inches from the middle of the bottom bracket to the top of the seat tube. 29er is referring to the size of the tires on the bike...29 inch.
Of course Dremer03 is spot on. In addition: There have been many posts explaining why frame size as in length-of-the-seat-post is misleading. Top tube length is a better indicator for fit and nothing beats a test ride... It takes a lot of experience to be able to assess the fit of a bike by just looking at the geometry of the frame.
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