Bruno,
the Summicron in LTM would be about the same size I think, the Heliar 2.0/50mm is a 39mm filter size as well. The Heliar might be a bit shorter, not certain there.
The Summilux definitely is bigger.
In the past a I shot a Color-Skopar 2.5/50mm which is the size (and design!) of the Summicron 35mm (!) lens, that lens was small, made from brass, and RAZOR sharp. I expect this Heliar to be at least as good as the Color-Skopar, since magazine tests proved it to be the 'sharpest lens ever tested'.
The Heliar is made from brass too, at least my new rigid nickel one is, not sure about the collapsible models.