If I were you, I'd fill out the map a lot more. Sure, the Soviet Union will be huge, it should be after all. What you need to balance it with, instead of having an equal number of cities for the nations is having more natural resources available to some, a larger populations for others, or both.
For instance, the US should have a larger population than the Soviet Union, helping to balance out the larger land mass. Also, most of the Civs on the map should be allies of the Americans, at least Japan and the UK should be mighty friendly with the US helping to increase her relative power according to the Soviets.
China and India need to have huge populations and China needs fairly abundent natural resources, as does the Soviet Union, the US, and Canada.
Depending on how you want to do it, you could make China allies or enemeis of the Soviets. Either would be consistant with the last hundred years. If they are hostile or neutral, though, it allows us to "play the China card."
Britian needs to be a major world player, give it a large population and plentiful resources.
France too should have plentiful resources and population.
Really, in general, you can render small nations nearly equal in power with the larger by giving them more resources and more people.