Bike racers (on the road) are running 700c wheels (~27") because they work best for road races. Tri-guys and some racers will occasionally play with 650c wheels but that is still 26". A 20" wheel is not something we are gonna see in bicycle racing, at least not on the road.
All below are 20" or smaller see photos:
http://www.moultonbicycles.co.uk/heritage.html
1985-1986 - In the Autumn of 1985, at the International Human Powered Speed Championships, a fully-faired Moulton ridden by Jim Glover broke the 200 metres flying start speed record, at 50.21mph (80.79kph). Then on August 29th 1986, at the same event, he broke his own record at a speed of 51.29mph (82.54kph) which still stands today for the conventional riding position. The Moulton ridden was an AM SPEED with the fairing designed by Doug Milliken of Buffalo, NY.
1987-88 - Dave Bogdan rode the ultra-endurance Race Across America (RAAM) in 1987 on a pre-production AM-Jubilee, and in 1988 on a prototype AM SPEED. His 1987 time was 11 days, 8 hours and 2 minutes. In 1988 he improved on this by completing the 3,073m (4,944km) route in 10 days and 15 hours and 1 minute, averaging 289 miles (465 km) per day. He finished 8th out of 35 starters in what is not only the toughest, longest 'single stage' race in the world, but also the ultimate test of man and machine.
1989- Richard Grisgby of the Bath Cycling Club, riding an AM SPEED, came first out of 180 in the 1990 Fowey Triathlon in Cornwall. He also came third in the cycling split of the Kingswood Triathlon out of some 400 competitors and was only 9 seconds behind a 1st category roadman.
1991 - Gerry Tatrai wins the 24 hour HPV race around the Milwaukee Mile Track, Wisconsin USA.
2001 - Dan Farrell (on NS SPEED) completes the world's longest randonnee, the 1400km London-Edinburgh-London in 100 hours.
2003 - Chloe Williams (on NS SPEED) completes Paris-Brest-Paris, a 1200km randonnee, as youngest female finisher and wins the Audax UK Merit Trophy for most outstanding cycling performance of the year. Dan Farrell rides PBP on prototype TSR.
2007 - Kazuhiro Yamamoto on a NS Double Pylon finishes 6th out of 320 in a 4 hour solo class at Suzuka, Japan.