Joy, thank you for your advice, especially about how the sweater will be stretchy when wet. I do want the sweater to shrink just a tiny bit, so I will use water that is barely warm. I do have a merino wool turtleneck from last winter that I washed in warm water and that shrunk at least a half a size. So I will have to use a temperature somewhere between cold and what I used for that sweater. And I do the dance too, when I hand wash clothes.
Dashielle, thanks for the caveat about the fine-knit sweaters. I think this one would qualify, so I'll be careful.
Vicki, thanks for backing up the warm water idea. I'll give it a go!