Here is the C117 CLA technical schematic:
Here is the W204 C-Class technical schematic:
The CLA actually is the longer car of the two but the wheelbase is slightly shorter. Due to the "four door coupe" styling the rear in the CLA offers substantially less headroom. The front seat though offers more headroom being positioned lower. The curb weight for the CLA is 3262 pounds in front wheel drive trim. This is considerably less than the W204 C250 which weighs in at 3515 pounds. Expect this weight advantage to be mitigated considerably with the addition of all wheel drive.
The other interesting aspect regarding the CLA is just how slippery the design is. How about a cd (coefficient of drag) of .24? Absolutely incredible and improves on the .26-.32 (depending on model) as measured in the W204.
Expect the new W205 to be a bit bigger, bit more expensive, and a bit heavier. It's moving upscale and the CLA is designed to take that entry level spot which was once held by the C-Class. It's a worthy model and with an the AMG version coming going to be a very exciting platform for the Mercedes enthusiast especially with its turbocharged engine.