The Defender 5500 sunglasses work great for me. They fit over my prescription glasses comfortably without being too huge even when using my full face helmet. I tried several other brands but these worked the best.
Size is small, measured my wifes and I heads and choose a size based on the chart, lo and behold they are too small. The price was great, they look great, but ... . As of yet haven't tried to exchange them for different sizes as it seems a bit of a hassle. Most likely will give them away and re-purchase another size and maybe another brand.