Since the euro sign normally occurs in conjunction with numbers and other textual information, it should stylistically match the font face and font size. There is no guarantee that it will, of course. You would have to make a guess at typical font size and use a suitably-sized image. Using an image which is 18 pixels tall might be a reasonable guess. That way  might look tolerable even within running text in a typical size, without changing line spacing too badly. (Typical normal font size is smaller than 18 pixels, but if you reduce the size, the image becomes more obscure and hard to read in many situations.) It does not pay off to try to set the font size to a specific value to make it match your euro sign image size. That would be backwards thinking and cause much more serious problems than it might possibly solve.