We've asked that, in a future release, hoop sizes will lock comparitive to the garment dimensions (along with a note saying something like "for visualisation only, not to exact scale") in the same way that decoration areas scale compared to 1st one set.
That way we can hopefully avoid undue expectations when the embroidery area appears to cover the whole of the garment even though it's just been a badly set-up 15cm hoop
For each product you can actually change the decoration area size and placement. For 15cm hoop you would make your area 4.5"x4.5" and select the circle mask lock the size and scale to the garment in the area you desire. Does not have to be exact.