> Inflation is size of economy / money supply.No, inflation is either an increase in the money supply (according to classical, i.e., pre-Keynesian, economics) or an increase in the average price level, often qualified to be the average price level of "wage goods" (according to Keynesian economics).
The underlying argument behind "size of economy/money supply" is that if the economy is growing (more precisely, if the rate of economic activity is growing), then increasing the money supply will not increase the average price level. In fact, if for whatever reason (and Keynesians have no trouble coming up with plausible-sounding reasons) you want the average price level to remain basically the same, you should print more money as the rate of economic activity increases.
While these are true statements, they ignore the fact that printing more money is economically equivalent to a wealth transfer from whoever does not get the newly printed money to whoever does get it (which under our current system is banks and other financial institutions). In other words, it's the same economically as a tax on ordinary people whose proceeds go to rich bankers and financiers. But it's much more palatable politically, which is why it's our current system. (The old system, before the Federal Reserve and other central banks gained power, was that when governments got in financial trouble they had to explicitly go to rich bankers and financiers and ask for bailouts, which was politically unpalatable, not to mention a huge pain for the rich bankers and financiers since people would see that they were effectively profiting from the economic woes of others. So the rich bankers and financiers got together and sold the politicians a system of central banks where all of this could be done under the radar so the ordinary taxpayers wouldn't see it and wouldn't complain about it.)