Aleppo (حلب) is the largest city in Syria - population 2.1 million.
Aleppo is a fairly conservative city. Dress appropriately to avoid any problems and to avoid standing out too much. Men should wear shirts and trousers and women should not wear anything that is too revealing. If you follow that simple dress code you should not have any problems. Headscarves are not necessary unless you want to enter a mosque but even then grey robes are available at no extra charge. There are no hostile feelings towards Americans or Westerners in general (although Americans tend to be subjected to more scrutiny by the authorities than other nationalities). You could, however, find yourself in trouble if you engage in open criticism of and against the Syrian government or the president. Your best bet is to avoid political conversations all together just to avoid any possible problems. If you do engage in political discussions with Syrians, be aware that they might face intense questioning by the secret police if you are overheard. As a general rule, always assume that you are being watched by the plain-clothes policemen (mukhabarat). You will notice that not many uniformed policemen can be seen in the streets, but this is because the police have a wide network of plain-clothes officers and informants.