In reply to tony:
I stuff mine into a compression dry bag and it’s about 1.5l fully compressed (13cm diameter x 12cm height in the bag I use). It’ll probably go a bit smaller with a smaller stuff sack. I’ve never stuffed it into the pocket, so don’t know how that compares. That’s for a size medium, bought in 2011, don’t know if OMM has changed sizing since then.
I agree it’s a great top, the best warmth/weight ratio of any non-down top I’ve tried. I’ve found it ideal for lightweight backpacking/bivvying (camp/tent/sleeping wear etc), as a back-up layer for winter long distance races and as a general spare layer for stops when hillwalking etc.
I find it way too warm to use when moving and it gets clammy easily, even when temperatures have been around -10°C but I run warm generally, so might be fine for you. I also don’t think the outer fabric would survive long directly under a rucksack but probably ok with something over it.
Hope that helps.