Sheep Garden JSK/OP sizing question

I'm considering buying Angelic Pretty's Sheep Garden (in either blue or black CAN'T MAKE MY MIND UP DX) but I wasn't sure if I should get the OP or the JSK because of the size. According to the site the JSK is 71CM at the waist but only has partial shirring and the OP is 69.5 but has full shirring.

So, my question is, how much does partial shirring and full shirring stretch? How many extra cm could you get out of the these dress with the shirring?

Sorry if this is common knowledge. Couldn't find anything about it. :(