This is an older vintage or antique lace piece that would have been used to make a nightgown or bed jacket. It includes the armholes, extends across the back and has little flaps in the front to meet in the middle.
It is approximately a modern size 6-8, but the arm holes are on the small side. It does have a few light stains.
Arm hole opening is 7.5 inches laid flat, about 22.5 inches across when laid flat, and about 9 inches high overall.