Anyone who has bought ready-to-wear clothes from more than one shop will appreciate that clothing size is a pretty flexible thing as there is not (as far as I can tell) an agreed standard for what each clothing size actually measures.
I highly recommend that you measure yourself before buying!
For reworked garments, the size on the product listing is the size given by the brand that made the original garment. The actual measurements for each garment are included in the product listing so that you can gauge before you buy whether it is likely to be a good fit for your body shape.
For handmade garments, here are the measurements that I have used for each size.
QUICK TIPS FOR MEASURING
Make sure the tape measure is straight around the body!
- Bust: measure around the fullest part of the bust (usually where the nipple sits)
- Waist: measure around the smallest part of the torso beween bust and hip (usually just above the belly button)
- Hips: measure around the fullest part of the bottom or thigh (standard patterns usually take this meausrement around 8 inches / 20 cm below the waist)
For more detail check out this video from Made to Sew. You can fast forward to these sections for specific measurements: