Micro SIMs in Germany

[ms93]

Wikipedia says:

Even smaller device sizes have prompted the development of a smaller still card size, the 3FF (Mini SIM) or Micro SIM. Micro SIM cards have the same thickness and contact arrangement again, but the length and width are further reduced to 15 mm × 12 mm. The specifications for the 3FF or Micro SIM also include additional functionality beyond changing the physical card size. [...]
The micro-SIM was created with backwards compatibility in mind. The major issue with backwards compatibility was the contact area of the chip. Retaining the same contact area allows the micro-SIM to be compatible with the previous, larger SIM readers through the use of plastic cutout surrounds. The micro-SIM was also designed to run at the same speed (5 MHz) as the previous version.

So I think there won't be problem after cutting it. Just try to buy any normal SIM in US before leaving, cut it and see if it works - if yes, you can do the same in Germany.