If you click on the "Size Chart" button above the size selection, you can see what the company considers its sizes to fit.
For size 18, it looks like you could go with either size.
If you know your hip measurement, it does say 1x/2x can fit up to 48" hips.
3x/4x can fit 50-56" hips.
You also might want to read some reviews- people occasionally will write what they find the size to actually fit.