Unfortunately, this depends on the font you are using.
With Times, the difference is rather small, but if you
change e.g. to Palatino, a comma is noticeably larger
than a quote.
but I think I'll stay with ,, '', it looks more german
than " ". (I have an aversion againt " because I see it
too often instead of reall quotation marks.)
How about using UTF-8 quotes, as in „German-style“?
UTF-8 supplies characters (not lookalikes) for all
kinds of quotes one can think about, and with the X
Window system, it is not difficult to get them onto
one's keyboard as well.