According to exact measurements on the size guide, I'm a size XS, but I usually want my tights to fit more comfortably (not pressing down on my stomach) so I was going for a size S, but reviews said it's a smaller fit so I went for a size M.
It was still too small lol soo tight. Especially around the thighs (I would say my thighs are average, and if yours are built then it'd definitely be a squeeze) and even around the waist where I allowed for 2 sizes up compared to my actual measurement.
As for the invisible scrunch, I had high hopes but it's not great :') maybe if I had more glutes lol but I barely noticed a difference. Maybe because I went 2 sizes up? But it was still wayy too tight on my waist so I wouldn't have been able to size down anyway.
In short, I will be returning but hope this helps people with sizing :) If you like a bit of room on your tights (don't want a super compressive fit) then get a few sizes up, but the scrunch effect will be reduced.