Sometimes you can block the sweater into the right shape. Here's how:
I always used my mother's ironing board, much to her disgust. So pin the sweater [half the sweater at a time] edges to the board - this means stick a straight pin through the edge of the sweater through the ironingboard cover. Space these pins about 1/2" to 1" apart, making sure that the sweater measurements are what you want them to be - this may mean pulling or stretching or bunching a bit. Then wet a linen or cotton woven dishtowel and put it over the pinned part of your sweater. Press on top of the wet towel with a steam iron, just enough to "set" the sweater beneath.
Remove the pins, repeat the above with the other half of the sweater. Then lay the newly-blocked and damp-hot sweater out on a smooth surface [I used the top of our chest freezer] and let it dry/cool overnight - again, make sure you have it in the measurements you want.
Good luck! And good for you for even finishing such a project - before Christmas, too!