It is often said that breeds with wide hips are better suited for sex. I think GSDs do not fall in that category. But there is plenty of videos of people comfortably having sex with them without signs of discomfort for the dog.
But as you will notice from other female dog threads, fitting in is not the only problem.
Some are not into sex with people, some are into it only sometimes, there are also health risks for the dog if you are not careful. I suggest reading more about it in the how to section.
There is also size variation within the breed. I have seen pretty large gsds as well ass some smaller ones and females are often on the smaller side so even if you get a positive answer here, you do not know what kind of dog you will get.