Buying a Rago Girdle

Hi girls, I've always wanted to purchase a Rago girdle. However I want to make sure the sizing is correct (or the slimming, smoothing effect will be wasted) When looking at the Rago size chart should I be buying on my waist size (34') even though my hips will have plenty of room, or should I buy a smaller waist size? Also do Rago sizes generally run smaller or bigger?

Thanks, Pip