With most glasses you can read the frame measurements from one of the temple pieces. For example, mine are 43/21/143. I like small frames because my eyes are close together (PD is 59). The 43 (mm) is the width of the lens. The 21 is the width of the bridge. And, the 143 is the length of the temple. The bridge width is less important to get right if the frames have nose pads, but if they are plastic it is very important. The temple length just needs to be at least "long enough." A little longer can be dealt with through adjustment. The lens size is a personal choice. I see that Zenni's site allows you to upload a photo and "try on" different frames - a nice feature.
Still, for glasses I wear everyday, I don't know that I could make a choice without actually trying them on. My current favorite frames are Microshapes, Skip, from Japan. They are non-flex titanium and took quite a lot of adjustment to be comfortable. They are incredibly durable. I send them off to eyeglasslensdirect when I need new lenses as I have two spare pairs.
Zenni would be great for kids who are tough on their glasses, adults who want a spare pair, or maybe sunglasses. Also, if money is very tight, it offers a way to get fashionable frames at affordable prices.