1/2" will carry everything to the back end of the car plus a few extra beyond stock, fitting neatly in the door sill valley. It can also be used under dash for some of the separate runs although a smaller size would work better for many. You will have a bit of larger bundles, perhaps, where a small amount of a larger size would be nice but it may be wise to not fully encapsulate those portions as they are under dash and never really in sight. Just use real wire cinch tape (not electrical) to bind them together every few inches.
When I did my mustang I bought 50' of 1/2", 25' of 3/8" and 3/4", if I recall correctly. I may have bought a bit more of something along the way. Been a while. I do know I have a fair amount of the 3/4" left over and some of the 3/8". It'll get used up here soon on the truck.
