It's already been verified that a 4.6" ICS screen can fit onto the same sized phone that fits current 4.3" screens (because ICS loses the soft-touch/capacitive buttons, which frees up screen real-estate).
Nexus Prime - Page 14 - Android Forums
We don't know for a fact that that's what's going to happen, but it is at least possible.
How is that gonna work on a phone? Is it like Honeycomb? That will be interesting...