joeblow7777
macrumors 604
The biggest issues I've ever seen mentioned are cancer (applies to all cell phones, though...jury is out), emotional problems (attachment to phone, avoidance of humans), vision issues (same as computer eye syndrome), and "text neck." That's neck/shoulder/arm/hand pain associated with texting posture.
All these are avoidable, of course.
If by "avoidable" you mean entirely unsubstantiated, then yes, they are.
keeps a 5S size as an option – the single biggest factor that has me think otherwise, is the keyboard. It's not necessarily the fault of anything other than my stubby fingers, and the screen simply being too small. Swype/Flex/Swift whatever have all been tried – I simply battle too much typing on the 5S. This alone has me thinking that once I go with a 6, I will also never be able to go back to anything smaller...