I'm a 38 G and ordered the XXL. It fit well on the tightest setting, and I feel like I could have went down a size. I tried it on first with a molded cup strapless bra, and although the lift was there, it pushed in too hard on the sides and gave it a funny shape at the bottom. I could have gotten away with it for my costume, but I ended up ordering a softer balconette bra that I thought would work better with the costume, and it doesn't push in the sides on that one.

I'm giving this four stars, even though I'm returning it. It's really well constructed and it definitely boosts cleavage as promised (see before and after photos) and at $12 it's a bargain. Unfortunately, I'm getting what I call side boob/side fat from where the strap cuts into my back, especially near my armpits (see pics). I personally find that to be really unflattering so I don't keep any bras that do that. I can definitely see this working really well on a leaner torso/physique. For reference, I'm a 36DDD.
Also, I found the top straps to be too short, even at their most extended setting, when adjusted, so the middle part in the back rides up too high (see last pic).

This is not a comfortable garment, but it isn't really supposed to be. The construction seems sturdy but you're probably going to feel squished and a bit restricted. What this garment does, and does well, is give you AMAZING cleavage. I never have cleavage due to my ribcage anatomy, even though I'm anywhere from a FF-G cup size with a 32-34 band (fluctuates, I'm also pregnant, lol). I ordered the large because I was nervous about a medium being too small, especially right now. I do wear some Small sized clothing but I think it's better to go up at least a size for this one, more if your cup size is really big. The large fits nicely, I included one photo of it on all the loosest hooks/strap settings, while the other picture is with the hooks and straps fully tightened. So, there is a lot of adjustability. I might buy this in a medium as well, especially after done with my pregnancy. I think it just kind of depends on what you are looking for. The smaller size probably rockets those boobs up even higher. I didn't have a costume or anything in mind, I was really just curious to see what it could do! Husband appreciates this experiment.

This did exactly what I needed it to/ for was perfect and price was excellent - almost didn't buy because of some of the reviews - but so glad I did!

Runs small but really lifts the girls. Is good for back support if your a sloucher. Order one size up from what you think you need.