If I measure a closet door opening, to be 35.5" x 80" and the door opening does not have any trim, what size bi-fold closet door would I need to purchase.
Beth, If your referring to an opening not intended to ever have trim, consult the rough opening (RO) parameters of the bi-fold door manufacturer, as the type and placement of the hinge pins and the thickness of the door will determine the space needed between the outer edge of the closed door and the frame of opening.
Most bi-fold doors are about 3/4"-1-1/4" thick (depending on if they're made of PVC or wood and solid or slatted...older, reclaimed doors are thicker). If the hinge pins are set half a thickness in from the outer side edge of the door, the clearance will need to be about 75% of the door thickness...especially if the door frame isn't perfectly square or hinge pins aren't set perfectly perpendicular to each other.
I'm working on a set of bi-folds that are 1-1/8" thick and require a min. 1/2" clearance between door and door frame (which doesn't give much leeway for error); where the doors meet there isn't supposed to be any space...they're meant to be flush to each other...but I'm adding an astragal, to one of the doors, to make installation easier and look more finished.
If you the doorway opening will, eventually, have trim decrease the RO width by 1-1/2" (assuming trim will be 3/4" thick each side).
If you know the floor is level; with no rug in the way, I'd give 3/4" for a space in top and bottom...some like it 1/2" but in adjusting the doors, after installation, sometimes that extra 1/4" is useful.