You can but the mixture will be less thickly spread in the 9 x 13 pan. You may want to increase the amount of batter that you use (assuming you're making a cake or something that spreads to fill the...
Avon's silicone Heart Bundt pan/mold is 9" across by 3.5" deep. Bundt cake pans come in several sizes, from 8" to 10" across, so the Avon pan seems about average size.