Are there certain clothing laws in china? (Beijing and Xian especially?)?

I'm going to China in a few weeks, and I was wondering if there are any clothing laws, or clothes which it would be considered inappropriate to wear there? Seeing as I am going in summer, I planned on just packing some skirts, shorts and t-shirts. However the skirts and shorts I am slightly worried about: is there any etiquette on how short they should be? I haven't found anything in my online search regarding this issue, and a few people have assured me that as tourist areas these places are probably used to western clothing, but I just wanted to check?