"TSA now allows small snow globes in carry-on luggage when packed in a passenger's plastic 3.4 oz bag. Snow globes that appear to contain less than 3.4 ounces (approximately tennis-ball size) will be permitted if the entire snow globe, including the base, is able to fit in the same one clear, plastic, quart-sized, re-sealable bag as a passenger’s other liquids."
So if you have a snow globe bigger than that, it's best to mail it home. If you buy it at the parks or at a Disney store, like the one in Downtown Disney, they will ship almost anything for you so that you don't have to worry about weight limits for your suitcase!
*This has been a Public Service Announcement from Dismerch.*