Daniel, just because a single currency is desirable does not mean the cost of it is worth the benefits.
ItalUK, giving power to Eurocrats is scary, so is dentistry or cancer surgery.
I must say I have no time at all for the "small countries will be crushed" idea.
Any number of smaller economies are doing well, Switzerland, Singapore, South Korea, etc.
I see no particular correlation, positive or negative between size of currency bloc and economic growth, except of course that successful economies become bigger. China grows quite nicely in a globalised world with an economy about the size of Germany, Japan which is not all that much smaller doesn't really grow at all. Euro zone growth before the crisis was pretty poor, it's now held up by the Germans.
I think that rishab may be referring to Euro zone countries, not the EU. Britain is not a member of the Euro and although its reserves are quite a lot lower than Japan, add to the EU total considerably.
Also, I don't see large reserves as a positive thing.
Stability is worth paying for but Japan's problem for most of the last 10-15 years have been too much stability, not too little.
Also currency reserves let governments play with exchange rates, rarely does this make a country richer, its just too tempting to play with.