Have to say, I'm not sure where the N5 = 5" screen comes from. The 4 isn't 4", the 3rd Nexus wasn't 3" .. and of course, the original Nexus didn't have a 1" screen Plus, the iPhone 5 doesn't have a 5" screen ..
Blowing aside the iPhone chaff (it uses a different numbering system) and considering only the current "Nexus x" models (where "x" is a number) then the "x" is an indicator of the screen size - not exact of course, but 4.-something in the case of the Nexus 4. Similarly the Nexus 7, Nexus 10 relate to the screen size of each device.
Perhaps Google will change its mind and start to use the number as a generation indicator like Apple does with the iPhone, but the new Nexus 7 (still 7, note) tells me that this is unlikely unless the screen size changes.
There again some marketing droid may decide that the existing screen is near enough 5" to call it a Nexus 5 anyway without changing a thing. We'll have to wait and see :-)