Good road bike or mountain bike sizing, together with the correct bicycle fitting, allows for greater cycling speed and comfort. There is a lot of advice about bicycle fitting as the videos show. The list order starts from the basic and works up to more complex bicycle fitting techniques. Even when buying a cheap bicycle, getting the basic bicycle sizing right will help enormously.
Doug LaFavor - bicycle designer by day for Kona Bikes, is Kona's Dr Dew. These webisodes are heaps of fun as well as being informative (with a good dose of brand development thrown in) My overall favourite is Episode 3, which stars Kona Clump team legend, John Cowan.