Originally Posted by
Moisture
Definetly wouldn't want to go longer, you can probably get away with 175mm, but its not worth it.
Going short is also bad. My inseam is 34.6 . This puts me at 190mm. My knees simply dread the idea of ever going back to 175mm. I would have been forced to stop biking if I continued on with the wrong size cranks. You're getting a nice smooth spin with a full range of motion by using the right size crank arms, and this will give you a far better idea of what sort of gearing will work best for you needs.
Now, for 5'7, a 50(cm?) Frame might be a little small for you. What sort of bike is it? Some pics of your frame fit would be helpful. Before you decide for certain that you need a different gear, make sure that you're riding with an optimized frame fit.
It's a Surly Cross Check with the notoriously long top tube. I actually had a 52 first. I could never get it quite right (with drops or flat bars - you should see my stem collection). I swapped the frame for a 50 and immediately felt more comfy, can make it work with drops or flats now too. Aside from overall comfort, I like the smaller size for the off-road handling in the rough stuff. I went with a 50 on my 650B geared gravel rig too (drop bar) - was also a little inbetween 50 and 52 when I tested them, but again I ride this one like a mountain bike for the most part, so I like the more compact frame.