I weigh 100 jin (about 110 Ibs / 50 kg), and I ordered size M. To achieve the look shown in the seller's pictures, you'd need to order size S. I'm quite satisfied with the quality and fit of the clothes.
Wore these all day and if I'm being honest yes they are comfy, but if you're not used to wearing thong flip-flops, you may have a problem with the rubbing in between the toes.