simple answer: You will need a bike of the exact category of your Lernfahrerausweis. So if you do the 125cc test, you will need a 125cc bike. If you do the "A limited" or whatever it is called, the bike needs to have an engine larger than 125 cc and I believe certain minimum power and not more that the 24(?) kw.

I do the unlimited test, so I needed a bike with more than 35kw/48hp...