@parrisk:
there is no standard-size.
it depends on state of country, region etc...
there is only a maximum-size.
for example:
the shortest possible is "D-D 1" for City Duesseldorf oder "B-B 1" for Berlin - in other regions and/or cities it could be for example "SHG-GH 1111" which is the biggest possible size (Region: Stadthagen/Germany) ... My Sign is for example H-XX XX (X is a mark for a number or a letter, i will not publish my sign

) I Live in City of Hannover and there is no shorter possibility at the moment. In Region of Hannover (not the City...) shorter Signs are possible.
It still depends on your luck, whether you can get a shorter.
But there are more things which increase or decrease your sign: do you want a "saisonal sign" (which is valid for example from april to november)? if yes, your sign has a mark of these months. (04/11) on the right side. if you have still space on your sign your lucky, otherwise you get a bigger sign.
but as written above: the maximum size is regulated.
if you still have any questions: ask!
(the above written text is for germany: other european countries have other signs. the netherlands for example have really small signs...)