I just bought a killer winter coat on sale, but it's one size too big. (They were sold out of my exact size.) Can coats be adjusted a full size by a tailor?
No. There is too much to be done. The most difficult thing for a tailor to alter on any suit-like garment (that is, any garment that is considered "constructed" because it has some kind of stiffening inside it) is the shoulders. And the shoulders are the crux of the fit. If they don't fit perfectly, nothing else matters. The waist can be nipped, the length can be taken up (although even that is a big job that most tailors will discourage because it affects where the pockets sit), but the shoulders can't be changed without taking the whole thing apart, an exercise that might cost you more than your money-saving coat did.
I also recommend against wearing the large coat as is and hoping no one will notice: Overcoats are of medium length and fairly fitted in current fashion, so it's the wrong time to try the romantic hobo look. (I did it myself for years with giant vintage-store overcoats, to avoid the horrifying investment of a new one, and I fear that they just made me look small.) Extreme sales are a dangerous thing: They can ruin your wardrobe. Next time, try to contain your excitement at the discount.
Russell Smith's latest novel is Girl Crazy. Have a fashion question? E-mail firstname.lastname@example.org.