I have a pair of prescription sunglasses that partially wrap around my face, but when I ride (mountain biking and commuting), dust and debris still end up in my eyes, and after twenty miles, my eyes are watering. I am looking to get a pair of goggles that can fit over my glasses, but I don't want to order a pair that won't fit my head (I'm 5'3"), and I have yet to find a goggle review that mentions a small size. I have a cross-country helmet, so they'd be sitting directly on my face. Are there any recommendations? I am looking to get a pair for under fifty dollars.
I no longer bother with prescription sunglasses, I have multiple pairs of Uvex safety glasses that fit nicely over my glasses. There are multiple lenses that filter all UV, come in different colours, etc. I also have a pair with clear lenses that I use when it's particularly dusty or dirty. (e.g. I sometimes ride over a very busy bridge)
If you do go with some sort of safety glasses, you can either find someplace online that sells them or look for a local safety/industrial supply company.
Here's some more Uvex-specific information, I'm sure other brands would also work:
The glasses I use are the Uvex Genesis with Espresso Lenses. The Gold Mirror lens looks cooler (both are 100% UV) but the gold surface finish is more fragile. The lenses on these and many other safety glasses are interchangeable so if you want another colour or need a replacement, you don't need to buy the frame. OTOH, the frames are not expensive so it's possible to have multiple shades for different conditions. (The last pair I bought last year was about Can$35.)
Uvex has a smaller series called the Genesis Slim that might meet your need for a smaller pair of glasses. They don't have as many colour choices for lenses, but both of the "Grey" choices block 100% of UV. This page has PDFs of reference info: which tints are available for which frames, details of the different lens tints.
Order a pair of glasses with more coverage from zenni-optical. It's nice not to have to wear a goofy contraption over top of your existing glasses. You can get a pair of prescription goggle type for around 25-30 bucks that will do the job for you. Just try sorting by type, use goggles as the filter. You just need to give them the details of your prescription and they will make them for you and ship them pretty quickly.
Until then, you could pick up some cheap hardware store goggles, a pair of ski goggles, or moto-cross goggles and try them on for fit over your glasses.
They make over the glasses goggles most of which are pretty affordable, but ski goggles might be too big for your application?