Nominal GDP is used to measure country market sizes, sorry.
First google result already says different: http://www.businessdictionary.com/definition/market-size.htm...
I woud assume it's not a term like "revenue" but more a general word. It's also not smart for your own analysis to assume a single value for it. If you write a business case for instance, it's quite smart to summarize how you size the market for your product/service as well.
"China and Russia might be almost as close allies as the pre-Trump US and Europe. "
That completely underestimates the cultural ties US and 'Europe' has and overestimates the co-operation between parties who co-operate pragmatically from time to time but really can't trust or understand each other.
" US lacks such interdependencies with close neighbors it can survive better in a time like WW2 but it is harder in a long peace time like we currently have."
That's some extremely peculiar economic calculus.
A big problem for the US is that it is a litle off, while most other social/cultural areas are closely connected. For instance, China and Russia might be almost as close allies as the pre-Trump US and Europe. But because Russia and Europe are so close, in fact the business dependencies between both are quite high as well. So a lot of business happens in that exchange, with high values due to the political risks. Since the US lacks such interdependencies with close neighbors it can survive better in a time like WW2 but it is harder in a long peace time like we currently have.