I looked at Wikipedia and it seems that 300 megatons would be enough to start a mild nuclear winter.  I also found this story that I hadn't heard before:

"What would happen in the United States and other nations if there was a distant nuclear war or nuclear winter was triggered deliberately [see Box - How it Might Happen]? A preliminary analysis of USDA data indicates that stocks of food in pantries and supermarkets could feed U.S. residents for about 30 days, and stocks in warehouses another 60-90 days. After that, they would have to live on feed stocks, which might last a year with tight rationing. Such feed stocks are not well distributed, and converting them to human consumption would present processing problems. Other nations would be in much worse shape. FAO estimates world food reserves at about 33 days now."

snip

"An experimental test of the nuclear winter hypothesis nearly happened in 1971. At a diplomatic reception in Moscow, a Soviet diplomat approached an American diplomat and asked him "Would the United States stand by if we launch a nuclear attack on China?" The American immediately said, "No, we most certainly would not stand by!" The Russian was a little taken aback by this immediate and emphatic reply, and said "Perhaps you should check with your superiors on this." The American said, "I will, but I know what their answer will be!"

"The purpose of the Russian's question was to remove the ambiguity that arose after the split between the Soviet Union and the Peoples' Republic of China as to whether the U.S. nuclear umbrella, which protected all countries not in the Soviet camp, now extended to protect China. By his reply, the American said that it did. The Soviet Union was then in an advanced stage of preparation for a nuclear attack on China's military and industrial facilities, which would also have caused the death of at least 300 million Chinese. A few weeks later, a higher-level Russian official asked the same question of a higher-level American official and got the same answer. Finally, Leonid Breshnev asked the same question of Henry Kissinger. He got the same answer, and decided not to go through with it. Shortly thereafter, the Chinese found out how the U.S. saved them from nuclear attack, and on April 6, 1971, they invited the U.S. ping-pong team to Peking. The rest is history."

"This was probably the closest the world has come to a nuclear war since 1945, and it would not have involved nuclear detonations on the territory of the U.S. or its allies. The yield of the detonations would likely have exceeded 300 megatons, which would have been sufficient to cause at least a mild nuclear winter. However, we would not have been prepared at that time to study it properly."

http://www.the-spa.com/jon.roland/vri/nwaos.htm

Only Nixon can go to China, eh?