What does 'one size' mean?
'One size' garments come only in 1 size. This does not mean that 1 size fits all. Typically it will fit a dress size 8-14 in the standard ranges and 16-20 in the plus size ranges.

Rubber Wear Medium and Large.
The rubber wear is designed to be a tight fit, the Medium will suit sizes 10-12 and the large will suit sizes 14-16. Always use a little talc on the body and inside of the garment to help ease it on.

Plus Size Lingerie Chart.