What is the ideal smartphone screen size?

We've seen smartphone screen sizes evolve from small 2.4" squares found on the Palm Treo to the massive 4.3" screens found on newer devices like the HTC HD2. So, what's the ideal size?  There have been rumored reports that Apple may be releasing a smaller iPhone. The current iPhone has a 3.5" screen. What if that was reduced to say 2.8" or smaller? That probably wouldn't make a huge impact on younger users who don't need to use reading glasses, but should Apple create a larger iPhone for those who use reading glasses? At what point does a smartphone become too large or too small to be usable? 

This is a question I've been asking myself. I've used all sizes of smartphones in the past. I've worn them on belt cases and I've carried them in my pocket. I don't use my smartphone as a multimedia player, so I don't need a huge screen. I also don't need reading glasses (yet), so I'd probably prefer a smaller device.

Well, let's see what happens in the smartphone market this year. My prediction is that we'll see a mix of smaller and larger smartphones and the major manufacturers will need to decide which size is "optimal."