To be honest, car salesman are generally so anxious to make a sale that any one of the statements you listed would be understood.
But, technically speaking, here is what you'd probably say:
1-I know what size car I want to buy.
2-I want to buy two cars and I know the sizes I want to buy.
3-I know the sizes of the cars I want to buy.
4-I know what car size I need. (This one is a bit awkward, you would most likely say to the salesperson "The car size I need is....")
5 is an incorrect re-phrasing of #1 above.