You'll need a 8' or 8.5' wide unit unless you have a FIA car. The width of the hips on 427 cars is too wide to clear the door cables on 7' doors.
I use a 16' long trailer with plenty of room for extra stuff. Many guys use a 20' since it is kind of standard and will fit longer wheelbase cars.
8 x 16 or 8 x 20 will get the job done
Make sure you either get a dove tailed floor or one with drop axles to help with loading. Standard floor trailers require extra ramps to cut down the break-over angle when loading.