The base on our existing faucet is quite wide, compared to the faucets I've seen being sold on amazon.de (which list the diameter, etc. in their descriptions).

I measured the circumference of our existing tap's base, and it's 17.78 cm (converted to cm from 7 inches), so then from that I divided by pi / 3.14 and got a diameter of 5.662 cm.

Many of the faucets I've seen being sold have a much smaller diameter than that, and of course I can't really see how big the hole in the countertop is unless I remove the old faucet, which I'm not willing to do until the new one is here and is being installed. Being that we rent, cutting a hole is not an option. But hopefully the landlord will be willing to install a faucet of our choice if we purchase one. The problem is going to be finding one that will fit, I guess.