Question about free size?

I was planning on pre-ordering the Melty Chocolate one piece from Liz Lisa, but it says "free" as the size and I'm a bit of a larger person. I'm not plus size, just average or ever so slightly above average by western standards (probably obese in Japan considering how things are over there LOL), though people say I'm skinny even though I'm not so don't listen to them. More specifically, most of my fat is distributed to my thighs and butt. I'm also slightly taller than average, about 5'7" and a half.

With that said, I'm not sure what I should do or if I should look for a different clothing piece or brand altogether, since I've heard that Liz Lisa is basically the Brandy Melville of J-fashion.

Any advice? I don't wanna waste upwards of £80 on something that isn't going to fit.