A 6 inch PB360 would be *really* interesting if it used a 16x10 aspect ratio.
The width would be about the same (less than a quarter inch wider), so pocketablity wouldn't be impacted, but you'd get a taller (just over an inch), more paperback-ish display.
Touchscreen? Minimal value-add on a 5-incher, to be honest. WiFi? Maybe if the browser app can bring in regular (non-mobile) web sites. In other words: I'd be more interested in a 502.