Sizing for the longer tank dresses coming up for sale are as follows:

SMALL : (pictured on the model above) -  16" across the bust, 40" from shoulder to the highest level of the hem, just below the knees 

MEDIUM :  17" across, 41" from shoulder to hem

LARGE :  18" across, 42" from shoulder to hem

XL :  19" across, 43" from shoulder to hem

2X :  21" across the bust, 43" from shoulder to hem

3X :  23" across the bust, 46" from shoulder to hem

Sizing for each dress is posted in its description.