Stalingrad was a large city and the streets downtown were made like most large citys, from concrete. In the major parts of town the streets were very wide. I have seen pictures with streets that look to be 4 to 5 lanes across.
Now if you need streets outside of the major parts of town, hard packed and well traveled dirt will do. Some of the pictures I have seen from around the factorys looks to be dirt as well, but I could be wrong. The roads in the worker housing parts of town look to be dirt also, these areas were generally located around the factorys.
Hope this helps...