Dry clean them. Never wash. (or size up and then wash).
Washed mine in cold water and dried on "no heat" setting (just like I do with my other Luchino from last year) - it became a square that is about 2.5" - 3" shorter than it was. The width shrank too, but it is tolerable and expected.
Here's a photo. Blue one is the washed one from last year (or whenever the first batch shipped) - grey one is a new shrunken one.
Once again - just be careful, those are great shirts - but I guess water is not their friend.