If you have a flat nose, you can try sunglasses that has wide upper bridges so as to give you comfortable wearing. From this point, you can try some ray ban aviator sunglasses. Besides, to get a comfortable wearing, sunglasses with nose pads are recommended so that you can adjust it yourself in case you feel uncomfortable of wearing it. So, Just try, i believe you can find one suitable from so many sunglasses.
Personally, the flat nose won't affect your choices of sunglasses too much. If you really care about it. I suggest you choose oversized sunglasses which can make a big contrast in the part of your nose, so as to make your nose look smaller. Besides, you can also consider aviator style sunglasses. The design of the two bar of bridges on the nose may also compliment your nose. Nevertheless, you shall also consider your face shape and clothes style when you choose sunglasses. Just have a try. Good luck.