Yes, but isn't it also true that if you're allowed to use a car that is 3900mm long (as per the 2017 regs) and that, for example, you have a B-segment car that is 3950mm long and a C-segment car that is 4250mm long you would use the shorter one for competitive advantage (assuming the marketing bigwigs said either one is as good, you choose)?

I don't know much about the rules in the early 2000s but wasn't the Peugeot 206 seen as having an advantage by being the shortest and therefore more agile? They even had to homologate a 4000mm long version of the road car over the standard 3830mm car. Length has high-speed stability advantages but still Skoda decided to drop the long Octavia for the short Fabia, thought not with much success...

Point being, nowadays you build a WRC car with your smallest regular passenger car available/possible i.e. VW will choose the Polo over the Golf, the Scirocco wouldn't get a second thought and the Up! is ineligible, same goes with Toyota for Yaris, Auris, GT86, Aygo and Hyundai for i20, i30, Veloster, i10.