Were the rear sway bars standard on all 69 SS cars or just on special order suspensions? Is the correct size 7/8"?

No, not standard in fact not very many 69's had one.
Not sure about the size but I would *guess* 7/8"

In '69 there was standard, F40, and F41. The sway bar was part of the F41 HD suspension RPO.

7/8" is correct Dean.