I've been told by SAs to never, ever do anything more than spot clean, but I've both dry-cleaned and hand-washed with success. That being said, I highly doubt it's possible or a good idea to try to shrink the dress with washing.
When it comes to fit, it depends on how you want the dress to... fit. Some people like their HL a little less constricting, others like their HL to really suck everything in. Personally, I have dresses that are a little big (but not obviously - I had a little size dilemma when I first started my HL addiction) and dresses that literally hug every curve. I tend to wear the dresses with a looser fit for a more casual look. My blue ombre scoopneck (the one Bitharvest is looking for, I got a size M even though a size S would've worked just fine) is a favorite of mine for going out shopping. Really dresses down under a leather jacket and with some opaque tights and flats, imo.