While I am a fan of larger displays and would absolutely LOVE to see a 5.5" WP, I'm not sure how nicely it would run on a display that big. Android can be customized and skinned for phones of all shapes and sizes, WPs really only have one interface, whether it's on a 3.7" display for a 4.7" display, it's the same but with larger text, tiles, etc. The Titan for instance, while it is a nice device, isn't really optimized to run WP in the sense that everything looks oversized, and unoptimized for the larger display. If MS let manufacturers tweak the OS a bit more to accommodate a larger variety of screen sizes better, that'd be great.
Anyways, hopefully the Titan 3 will come in at around 5" like the rumored One X replacement. Obviously the larger screen size isn't for everyone, but I for one am a huge fan; the 4.3" display of the 8X is a bit of a letdown for me.