Certainly, this question leaves some things unclear. Personally, I would ask why someone would want to put an oriental rug under their bed? Why would you purchase an oriental rug, and leave it mostly hidden by the bed or other furniture?
Also, there can be issues with placing heavy furniture, such as a bed, on a handmade carpet, which can over time deteriorate the pile fibers, leaving you a worn-out work of art. Rugs and carpets made of natural fibers, such as wool and silk, can also be consumed by the larvae of the carpet moth or clothes moth, which prefers dark and undisturbed locations such as under your bed, which may rarely get vacuumed.
With these concerns mentioned, I do endorse the idea of furnishing one's bedroom with handmade rugs, but maybe in a different manner. Many rugs are suitable for bedside locations depending on the dimensions of your room. Often times a 2x4 or 3x5 foot rug will work along the bed or perhaps you have plenty of space for a 4x6 or larger rug at the foot of your bed. These locations permit both enjoyment of your rugs as well as preserving the life expectancy of your rug's pile. One other option is to hang a rug on the wall as if you're displaying a high-end work of art, which adds to the overall beauty of a room without taking up floor space. If you decide to disregard my advice, please at least protect your handmade carpet with a quality non-slip rug pad of a quarter inch minimum. And please check out, 5 Best Oriental rug store tips to make your rug survive for generations, where we discuss proper care for your Oriental rugs.
But you may also be searching for a machine-made manufactured rug or carpet, rather than a hand-woven rug or carpet. There are certainly some benefits to choosing manufactured rugs. They are usually more affordable. Synthetic fibers last longer, are more stain/mold resistant and don't attract pests. If you weren't entirely sure what size to purchase; I’d recommend at minimum 2 feet of carpet reaching beyond each side of the bed. If your bed is against a wall, consider reducing 2 ft. off the length so that the rug doesn't extend 4 ft. at the foot of your bed. If you have a larger room, consider purchasing a larger carpet where you can enjoy the comforts of a fully carpeted space.