I've owned two Zeiss 21mm f2.8 lenses - I sold the first one then missed it so much I bought another.
The f2.8 version is no bigger than my 50mm summilux pre-ASPH and, if memory serves, about the same size as the 35mm Biogon. Now, the only reason I opted for the 2.8 is for that extra speed in low light as I do shoot the occasional wedding on film still (I personally prefer it but hey, that's me, I'm weird like that
). The new CV that's all the rage (21mm f1.8) is probably about the same price but it's bigger and while I'm sure it's an awesome lens, I don't know if I want another "large" lens in my kit. I've tried to do that with the CV 50mm f1.1 and wasn't happy with it and yet I was perfectly happy with the CV 35mm f1.2 - go figure.
I currently don't have a digital full frame upon which to mount the lens but I have used it extensively on film. I'll hunt for a shot or two and post for you.