<<***Type of car (larger or smaller) for the two tracks in the country>>

Doesn't this question answer itself?

Diesel or petrol is a question of fuel prices and vehicular efficiency. Go investigate that and you'll have your answer. In the UK prices are or tend to be slightly more for diesel than petrol, so if the cars are equally efficient, the answer is obvious. In France, the equation works the other way - diesel is far less expensive.

It ain't the 'two tracks'(whatever that means) - it's the single track roads

But actually, single tracks aren't that big an issue -- there are frequent passing places. It is the narrow roads through towns/villages w/ cars parked every which way on both sides.

I would not get the very smallest class(es) of car because they are really REALLY small. But I would get the smallest that will accommodate however many you are and your (hopefully small) luggage.

As for diesel vs petrol-- diesel costs more but gets a bit better mileage, petrol costs less/lower mileage . . . so it really doesn't matter much - which is a good thing since most agencies won't/can't guarantee any specific model.

I've never heard of a diesel car being less efficient than the equivalent petrol car so the main advantage of diesel is the extra range per tankful which in the Highlands could be very useful.

alanRow is offering a good suggestion. Fuel is scarce enough in some parts of the Highlands that saying, "I'll fill up in the next town" may be a mistake.