The same way as having the seat in the UN's Security Council gives you the influence, but in the end France has little to say when compared with US or China.

Germany even without any representation in ECB will have much more to say than other countries. Because in the end it is a real political power that allows anyone to make decisions.

The point is that all of the countries in the EU are well developed countries with similar growth and closely tied economies. I can't imagine a monetary policy that would benefit one state while at the same time damaging the other one so much it would negate all the benefits of joining Eurozone.