As a matter of fact, Prada is an Italian fashion products maker located in Milan, they are know making a wide range of quality products. According to your description, the pair you bought is not what fits your head size. Typically, there are few ways to tackle this problem, except send them back and asking for another pair that is suitable to your size. Generally speaking, they would not charge you .
If you want to tight the prada sunglasses, you could adjust both the frame side and nose pad positions. You could use the screwdriver which may tight the screw of them. Then you could try on and see whether it is ok. The way is so easy. You could have a try.