'Part/Craft Dimension' indication: Diameter and Length should be more than enough to accurately size real rockets.
(At the moment I'm using different grid sizes and a 'counting method' to appropriately check lengths and also the rocket's shadow to estimate its diameter, but this way is far from ideal.)
'Length/Height' and 'Diameter' fields could be added to the 'Part Properties' display of the building window, along with weight and cost.
On the 'Staging Panel', the full craft dimensions could be also be helpful, displaying the widest diameter and the total height.