Big or small, it&#x27;s not just the size that matters. One per city would be almost always very unoptimal.<p>Eurozone is not good currency area because it&#x27;s too big, but because it does not fill the criteria for good currency area [1]. US is much better. US-Canada economic region might be optimal if divided into three[2], but it&#x27;s not a big deal. United States can function fine as a single currency region.<p>[1] A  Theory  of Optimum Currency Areas (classic paper from Mundell)
<a href="https:&#x2F;&#x2F;www.aeaweb.org&#x2F;aer&#x2F;top20&#x2F;51.4.657-665.pdf" rel="nofollow">https:&#x2F;&#x2F;www.aeaweb.org&#x2F;aer&#x2F;top20&#x2F;51.4.657-665.pdf</a><p>[2] Optimum Currency Areas within the US and 
Canada: a Data Analysis Approach <a href="http:&#x2F;&#x2F;utopia.duth.gr&#x2F;~pgkogkas&#x2F;duthwp&#x2F;4-2012.pdf" rel="nofollow">http:&#x2F;&#x2F;utopia.duth.gr&#x2F;~pgkogkas&#x2F;duthwp&#x2F;4-2012.pdf</a>