I have owned a Fjallraven Polar poncho for a few years.
In it's supplied stuff sack it is 25cm long and has a diameter of 12cm.
Squashed up it forms a rough ball of about 12-15cm.
Looking at Fjallraven's website, my old poncho has nowhere near the specs. of their newer model
so my data may not be of much use to you.
It has given me good service but is now a little leaky and I plan to apply Nikwax gear proofer to it.
Happy trails...torc.