I guess LEGO designers probably started with some existing elements like the size of the wheels? Or perhaps their marketing prescribed a certain piece count to derive at a target price for the set.
Your idea of 1:24 scale would probably be around 10 stud wide for the vehicle? The current city sets have vehicles around 6 studs wide and 8 studs for some of the trucks. Another vehicle I can think of is the Vampyre Hearse 9464. Have not built my set yet but it is 8 studs wide. Overall length is about 20 cm which is close to 1:24 scale.
I've never made a vehicle from scratch but I guess you could get a set of the Camper to use the parts as a base for the F350 or F450 ambulance. There's plenty of red and white pieces that is a suitable color combination for the ambulance.
Do take some photos of both the progress and completed model if you do attempt it. Cheers.
Wouldn't it be easier to design in 1:24 which would be twice the mini figure scale? I would think that the bricks would work better.
I just joined a volunteer ambulance corps and I am interested in making a larger size Ford F350 or F450 Ambulance. Those are the boxy ones with the cab out the front.