The Economist magazine has published its "World in 2009"
They have a forecast for China's GDP in 2009 of 4.8 trillion (8% GDP growth) and for the USA $14.8 trillion (-0.2% growth) and for Japan $5.39 trillion (0% GDP growth).
The US$4.8 trillion estimate for China in 2009 matches prior estimates from this site. The US and Japanese GDP estimates are less clear for 2009.
The bea.gov [bureau of economic analysis] has the 3rd quarter 2008 US current dollar GDP at $14.43 trillion.With slightly negative growth for the 3rd quarter and 4th quarter and likely overall negative in 2009.
IMF estimates for nominal GDP for 2009-2013 are listed at wikipedia. [The recent financial turmoil has caused large fluctuations in currency.]
The National Intelligence Council has forecasts for the world in 2025, but the forecasts appear to be retreading older estimates. Something like the Goldman Sachs paper from 2003 on the Brazil, Russia, India and China.
Japan's official statistics are for GDP at about 560 trillion yen [2000 chain linked].
Current price GDP seems to be less about 530 trillion yen.
The yen is about 94-95 to the US dollar. This places the japanese economy at about 5.5 to 6 trillion US dollars.