The USSR, A.K.A. The Soviet Union.
The second largest country that used to be part of the USSR is the Ukraine
Nue Zeeland
The largest country in 1980 in terms of land area was the Soviet Union.
Russia (USSR)
It was the USSR that broke up into 15 post-soviet states. The largest of these by far is Russia. Russia and USSR are not interchangable terms, Russia never broke up.
USSR
Tbilisi is the capital and largest city of Georgia (the country), which was formerly part of the USSR.
Strictly India. USSR is not a country. The boundary with China is disputed by India.
The world is round. There is no central country. China called itself "The Middle Kingdom." There is a point in China that is the most distant from the ocean than any other point on earth. Even though the USSR broke apart, Russia is still the largest country in land area of any country.
The USSR broke up in 1991 into 15 separate countries. They would all have been 1 single socialist country prior to that.
The USSR did not break up and become Russia again. Thinking Russia and the USSR were the same, is a common mistake. Russia was just part of the USSR, along with many other countries. Russia was the largest of the countries that were part of the USSR. In 1991, the USSR broke up and these countries, including Russia, all became independent.