I am not an expert in this, but here's what I found while pondering the
same issue. My american '84 XJS acceped a standard DIN size radio. There
is some concern about the depth into the console, though, as the used
radio I chose was actually too tight a fit. It extends out of the
console perhaps a half-inch, but looks fine (no ugly rough edges
showing) and doesn't interfere with the shift lever (although it might
interfere with CD insertion if the lever is in the "park" position). I
kept it anyway.

Most cars, at least here in the US and, I think, Germany (and possibly
the UK) have standard DIN or double-height DIN openings.

Here's a link with some info on standards:
http://stereos.about.com/od/introductiontocarstereo/a/wtlf_carstereo.htm

Here's another (look down the page at the "DIN-size" paragraphs.

http://tinyurl.com/98bkh

The used radio/CD player I purchased came with a sleeve that fit into a
DIN opening, and also a faceplate that dressed up the exterior edges. I
was able to look up some installation instructions for several radios
and discovered the wire colors were standardized, too, which allowed me
to easily solder the new radio wire harness to the old radio's pigtail
to make a plug-in adapter harness that didn't disturb the vehicle harness.

One more concern... if I recall correctly, my radio had a 45watt output,
and many newer car radio/cd players have much higher wattage, which
could present a fire danger to vehicle wiring (high current input
required) and endanger the speakers themselves, so I was careful to
choose a unit with nearly the same output as the old one (I think the
info was printed on a paper glued to the old radio) as I did not want to
get into rewiring the vehicle.