I make mine from scratch unless I am short on time then one of those big jumbo snack size ones come into play - here' s the recipe from Kellogg....
Kellogg's® Rice Krispies Treats® Original
This recipe has been tested and endorsed by Kelloggs® Rice Krispies®.
3 tablespoons margarine or butter
1 package (10 oz. about 40) regular marshmallows
or 4 cups miniature marshmallows
6 cups Rice Krispies®
1. Melt margarine in large saucepan over low heat. Add marshmallows and stir until completely melted. Remove from heat. 2. Add KELLOGG'S® RICE KRISPIES® cereal. Stir until well coated. 3. Using buttered spatula or waxed paper, press mixture evenly into 13 x 9 x 2-inch pan coated with cooking spray. Cut into 2-inch squares when cool. Best if served the same day. Yield: 24 squares MICROWAVE DIRECTIONS: In a large microwave safe bowl, heat margarine and marshmallows at HIGH for 3 minutes, stirring after 2 minutes. Stir until smooth. Follow steps 2 and 3 above. Microwave cooking times may vary.
For best results, use fresh marshmallows. 1 jar (7 oz.) marshmallow creme can be substituted for marshmallows. Diet, reduced calorie or tub margarine is not recommended. Store no more than two days in airtight container.
Hope this helps