I had basically the same situation in my 70's-built house, the closet doors went all the way to the roof, and were horrible looking bi-folds.
![before](https://i.stack.imgur.com/ECnNSl.jpg)
Basically what I did is framed in a header to bring the opening to 81" (remember to account for 1/2" drywall while framing):
![new framing](https://i.stack.imgur.com/yYbwVl.jpg)
Then some drywall and paint (note I didn't yet repaint the ceiling in this picture):
![drywall complete](https://i.stack.imgur.com/JhKoll.jpg)
I had 4 closets like this. One by the front door got sliding mirror doors, the rest got frosted-glass sliding doors. The right picture below is actually the "after" shot of the first one in this answer.
![finished with frosted glass doors](https://i.stack.imgur.com/Hle7km.jpg)
The width is going to be challenging for stock stuff. If it was a bit wider, you could probably split the door into two by putting a small column in the middle, but that would leave you with a couple 3.5' doors, which are pretty small. If you went to bi-folds, you might be able to do a 4' and a 2' door, though that might look a bit strange.
A quick search turns up some custom closet door manufacturers, which might be an option to get multi-panel sliding doors, like this:
![alt text](https://i.stack.imgur.com/ntVsV.jpg)
If you do this you can probably also get them 8' high, but honestly, even without the doors on, just making the opening 81" really updated the look of the house.
Yet another option is that you could shrink the width down to 72" (basically just extend the wall/framing on one side by 9"), and along with making the height 81", you're into a standard size where you can get both sliding or bi-fold doors off the shelf from any box store.
Good luck, and let us know how it turns out
Best Answer
although every situation is different, generally when you frame the opening, add:
horizontal 3/4" to each side for the jamb (or whatever the jamb thickness is) 1/2" to each side for the gap for shimming the jamb 1/4" extra for bifold or french doors
vertical 3/4" to the height for the jamb (or whatever the jamb thickness is) 1/2" for the shimming at the top 3/8" to the bottom for the gap between door and floor (we usually do 1/2", or 3/4" if there are carpets or matts being used in winter
so for a 72 x 80 bypass door, your RO will be approx 75 x 82. since its a bypass door and the floor could be out of level, i would do 75 x 82 1/2.
remember too that trim will cover a lot of stuff up, particularly now when most casings are in the 4" or wider range.