A 1960's Unimog may not be that large but those aren't what we're talking about. Most overland trucks you'll find in Africa and South America are modern cab-over 4x4 chassis (Unimog, other Merc models, Mann, Fuso, etc), in the range of 20ft (6m) long, 8ft (2.4m) wide, and 10ft (3m) high. The cutaway chassis alone is $50-75k new.