Simply add the width of both doors, plus clearance between the doors and finished opening trim plus the amount of gap between closed doors. You will also need some extra space between the trim and the framing to plumb the trim.

Example:

24" doors----------------------------------48"
One half inch door to trim per door-------- 1"
3/4" thick trim----------------------------1&1/2"
1/2" plumb clearance----------------------- 1"

Total width for rough opening is 51&1/2"

Height will be determined by the height of the doors and the desired clearance between the bottom of the doors and the floor. Many bifold doors are designed to fit 80" high openings and allow a half inch clearance at the top and 1&1/2" to 2" at the bottom. That means the top rough header should be at 81" to 82&1/2"

Buy the doors prehung before you make the opening. Then you can make the opening fit properly.

It's bi-pass closet doors that have a different rough opening than bi-fold or standard doors. Bi-folds are framed same as a typical interior door. For double 24" doors, you would add 5" for the header size. With the trimmer or jack studs installed, that gives you 50" for your width. 82" is typically standard for height, but as handy states, it's dependent on your floor type and how high or low you want the doors off of the floor.

24"x2 = 48" + 5" for the header - 3" for trimmer/jack studs,(1-1/2" each side), = 50" width rough opening.

80" doors plus jamb = apprx 81" plus desired off of floor height. For carpet. typically 3/4", tile typically 1/2", laminate typically 3/8", etc., including the desired clearance.

Here's a link that shows a typical framed opening with trimmer/jack studs supporting the header.

Here is a link that might be useful: bi-fold r.o.

A 5' bifold is sized to fit in a 60" wide opening. They are already trimmed a bit narrower than the stated size. For height, you need an 81' tall finished opening, IIRC. Depending on what finish you're using, you need to end up there, drywall-wrapped openings allow 5/8 finished thickness because the corner bead eats up an extra 1/8". Allow 1" for each side of the 3/4" jamb boards, but only allow 3/4" per side if you're using prefinished 5/8" jamb stock. This is a case where the more certain you are about your finishes the better off you'll be. Use studs free from all twist and crown for these frames.
All in all, unless you're drywall-wrapping, make the openings toward the larger side so you can pad them in if uncertain about jamb material. Better than removing jack studs for planing!
Casey