What would be a comparible size for this item?

The product page includes a size chart.

You can check that chart out here (it'll direct you to DreamGirl, which is the maker of the corset set.

I'm pretty sure it goes by bra band size.

Get the old tape measure out and take the measurements and then compare them to the chart.

This item doesn't come in Small, Medium etc. like most do though, so the sizing chart doesn't work for it.

the size charts don't always help haha

charts don't always help