Start Expanding That Garage For Detroit's Next Generation
US car manufacturers, inspired by popularity of big cars and trucks, plan even larger models with equally gigantic sticker prices; Ford Motor Co is testing 19-foot sport utility vehicle weighing more than three tons, Chrysler is rolling out new Dodge Durango sport utility wagon that is roomier and more than foot longer than biggest jeep, and General Motors is planning redesign of popular Suburban over next two years to make it more appealing to families; big vehicles are being snapped up by consumers who they think they are safer, who want to ride above traffic and intimidate others or simply want something bigger than what is owned by friends; safety groups are expressing concern about rise in collisions between such cars and smaller, lighter ones; diagram (M)