N97: 103cm3 --> 1.46 g/cm3
N810: 129 cm3 --> 1.75 g/cm3
Rover: 121 cm3 (-6%) --> 1.49 g/cm3 (-15%)

I'd say the Rover (while only slightly smaller than the N810 in volume) will feel like a mobile phone again, not like the massive solid thing that the N810 is, due to the lower build density (less metal, more plastic I'd guess...)

I currently own a N82 - the Rover will have about the same length, only 1cm more in width and be slightly thicker (about 25% larger in total). Sounds good

Speculation

So RX-51 is called that because it's length minus height is 51
111-59.7=51.3 (with rounding 51)
So RX-71 should be n810 sized screen (but less border area)
131-59.7 = 71

My old pocket pc has a 3.5" screen which has the same width as my 4.3" n800 screen, so the devices being the same width as the RX-51 would make sense.