A "wheelbase model" refers to the length between the front and rear wheels of a vehicle. It is used to describe different sizes or versions of a particular vehicle, as the wheelbase affects the overall length and stability of the vehicle.
The automaker is saying it's the only long wheelbase model in the «premium compact car» segment, in fact. (slashgear.com)
Here we see the medium wheelbase model with the low roof, but long and extended wheelbase configurations are available with two taller roof options. (cnet.com)
In some markets (depending on the manufacturer), short wheelbase models are excluded completely, and only long wheelbase variants are sold. (en.wikipedia.org)