Clothes sizing. Please help!

[KidnappedByPenguins]

I've seen a tunic top I'd like to order online (am abroad, returns awkward) This is the size chart they give, hopefully attached.
My measurements are 83/68.5/97
What size should I get?

You will need a 12 or it won't fit your hips.

Its going to be big on your top half though. I'm not sure it's a style that will fit you.

Nothing ever fits me! I've even been looking at free patterns to make myself something...even though I can't sew!
I was wondering if I could stitch up the side seams at the top to make the top part slightly narrower it might fit. Or is that a crazy idea?

It's 50:50 cotton/acrylic. Is that likely to shrink or lose its shape and get looser?