size

Which of these are correct:

1-I know what size of car I want to buy.
2-I want to buy two cars and I know what sizes of car I want to buy.
3-I know the sizes of the cars I want to buy.

4-I know what car size I need.

5-I know what car size I want to buy.
(This one sounds wrong to me because I am not buying the size)

[Ouisch]

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.