Q: For a man and specifically in Amman, Cairo, Damascus and Oman
A: Yeah, I'm originally from Cairo. Wearing a normal t-shirt is 100% perfectly fine (especially in Amman, Cairo, Damascus). Long khaki shorts below the knew would be considered acceptable. If you are a tourist, you won't be looked down upon. If your a local, it would see somewhat strange (i remember i wore shorts below the knew and a kid went passed me and said "The bottom part of ur pants are misssing" and he laughed) Culturally speaking it isn't an issue. Perhaps in Oman though it would be looked down upon.