I would choose a location that has direct viewing of a fireworks show.
If you are willing to spend the money, California Grill at the Contemporary & Narcoosees at the Grand Floridian would be excellent choices. CA Grill even has an outdoor catwalk area to view the fireworks, which would be a fabulous place to propose!!
If you are looking for someplace less fancy and more fun, you might check out 1900 Park Fare at the Grand Floridian. We had our Welcome Dinner there for our wedding. Cinderella, Prince Charming, the Wicked Stepmother & Wicked Stepsisters are there & they are quite entertaining. Especially for an even like yours, they would be sure to put on a hilarious show for you!
Otherwise, what are you looking for exactly & we can try to come up with a place that suits your needs:
-Entertaining & fun?
-Quiet & fancy?
-At a park or at a hotel? There are also some great options in Downtown Disney.
-Buffet or sit down? Any specific type of cuisine or just plain "american food"?