Depends on what you mean by 'woollen' / what kind of wool it is / how it has been treated..
..some stuff does not shrink at all (like icebreaker merino shirts for example, other things shrink to baby-size even at 30°C).
I think washing it to shrink 'half a size' is not going to work (or work too well), I'd rather try putting it in a dryer.
Depends on what you mean by 'woollen' / what kind of wool it is / how it has been treated..
..some stuff does not shrink at all (like icebreaker merino shirts for example, other things shrink to baby-size even at 30°C).
I think washing it to shrink 'half a size' is not going to work (or work too well), I'd rather try putting it in a dryer.