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
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.