SPANX Women's Thinstincts¿ Open-Bust Mid-Thigh Bodysuit Very Black Large

I am 5'8, 170 lbs and have had a child... hips 32 natural waist 26.5 and bust is small... .... by the size chart i should be a small?? But at 170lbs?

What does the back look like? My dress is a halter

Is the crotch open or not ?

Where can you size chart? I’m 5’4 and weigh 175 not sure what size I would need. Could you help me with this!