In the framing industry, the size of the artwork and the size of the frame are the same thing. A 24x36 inch print will go into a 24x36 inch frame
You measure the inside dimensions, not the outside. The outside will be determined by the width of the molding, not the size of the print.
If it is a one-inch molding, the overall dimensions will be 26 x 38. If it is a four-inch molding the overall dimensions will be 32x44 inch, but they will still come them both 24x36 inch frames because the artwork that it fists is 24x36 regardless of how wide the molding is.
The size rabbit is usually 1/4 of 3/8 because you can not put an exact 24x36 into an exact 24x36 inch frame. But they still call it a 24x36 frame.
The overall size of the artwork being framed will include any matting as well. So a 24x36 inch print with three-inch mat is now 30v42 and is going to need a 30x42 inch frame.