How could the UK cope outside of the EU?

How could the UK cope outside of the EU is a question sometimes posed by supporters of the REMAIN campaign. 

It's a good question, after all:

The UK is the world's 5th largest economy

The UK is a permanent member of the United Nations Security Council

The UK is a member of the G7

The UK is a founder member of NATO and one of the biggest contributors to NATO

The UK is a member of the the Commonwealth of Nations

It's a mystery how the UK would cope if it left the EU.