I don't have it but I share your frustration with the lack of dimension data in product descriptions from Faller these days.

You may be able to get the answer from Faller customer support though if nobody here has the answer.

I don't myself have that kit but it's an update on the former kit B-921 which was often used with the 'original' a-m-s car system (auto-motor-sport).



See this box image showing use with the a-m-s vehicles. These were touted as 'H0' but really they ranged anything from roughly 1:64 up to 1:76. Some vehicles were in better proportion than others but this was largely due to the constraints of the chassis and motor of that era.

Faller Car System vehicles are much closer to 1:87 so all vehicles will pass through the larger portal, and many of the smaller ones would go through the smaller one with no issues.
Someone may have B-921 they could measure exactly.

Cheers

F.Y.I.: I received a response from the Faller support!

A = 6,7 cm

The gateway predates cars - its likely for pedestrians and horses/carriages. But with European car and those roads, they could both be leveraged for vehicles... the smaller for cars and bicycles the larger for trucks even. ;-)