I posted this on another postcard thread...
This comes straight from the Disney website official rules so we should all send in 3.5 x 5.
HOW TO PARTICIPATE
Starting on September 18, 2006, eligible participants can enter without being present in a Disney Property by writing their full name, complete mailing address (including postal code), daytime telephone number (including area code), and date of birth on a 3.5"x5" postcard and sending it with proper postage affixed to: (i) if a legal resident of the United States, District of Columbia, Puerto Rico, Canada or Mexico: P.O. Box 8629, Elmhurst, IL 60126; or (ii) if a legal resident of the United Kingdom: Disney Dream Giveaway, 483 Green Lanes, London N13 4BS. Participants may enter once per day during the Giveaway Period. All mail-in entries must be mailed separately, postmarked by December 31, 2007 and received by January 14, 2008 or they will be void.
This is from the United States Post Office website...
Sizes for Cards
Cards (or postcards) are an inexpensive way to get an immediate message to customers. When cards arrive in the mail, theres the message -- no envelope to open! First-Class cards are a great value, too. With First-Class cards, you pay a low rate and get all of the benefits, like forwarding and return, that come with First-Class Mail. And, if you mail at the single-piece First-Class Mail rate, there is no extra work involved -- simply drop the cards in a collection box.
You may think that your mailpiece is a "card," because it is a single sheet of paper. But to qualify for mailing at First-Class Mail card rates, a card must be:
Rectangular
At least 3-½ inches high x 5 inches long x .007 inch thick
No more than 4-¼ inches high x 6 inches long x .016 inches thick