Great Vintage Condition! There is no brand or size tag; based on the measurements, I would say this is a 7/8. I listed more detailed measurements below. There are some stains on one back hem. Please see the photos.

When laying flat, the waist measures 11 inches and can stretch to 12.5 inches (elastic in back).
When flat, the hip area measures 13.5 inches.  
The inseam (crotch to the bottom of the leg) measures 19 inches. 
The rise (waist to crotch) is 10 inches.  
Waist to hem measures approximately 29 inches. 
The opening at the bottom of the legs measures 8 inches. 
This jumpsuit is in great vintage condition and comes from a smoke-free home. 
Thanks for looking!
List A872