With pre-made steel-boned corsets, you measure your waist and go down a couple inches. The corset you linked has plastic boning, so it isn't going to be able to affect your shape at all, so I'm not really sure what you'd do there. Personally, I'm a bit of a corset snob, and wouldn't dream of getting one without steel boning, just because if I'm wearing a corset, I want it to shape my body like corsets are supposed to!
Good luck though...sorry for my useless rant