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IME wheelchairs go pretty small.
Big wheeled ones have the wheels come off super easy, so you can put them seperate from the seat part.
Small wheeled ones tend to fold pretty flat but are quite tall (often the handles can fold down though)Previously my wife has driven with me (6'6") and either my grandma and the small wheeled chair, or my little sister and a big wheeled chair, in a 3 door Fiat Punto (me in the back)
Most cars are capable of transporting 4 adults, even the smaller ones.
Wheelchairs are fairly small and light, so can be thrown on a parcel shelf if absolutely necessary.If you are doing 200+ miles with luggage, go for bigger, but if it is an hour to the coast for a day, shoehorn them in to whatever fits your budget at that time.
Whats the cheapest car I could rent to get four adults in and with a boot that could fit a wheelchair for elderly relative? I have no idea how big the wheelchair might be.