1
That is the biggest dog I've ever seen.
Sign 'DOG', then point to it, followed by the sign for 'BIGGEST' to emphasize its size. Use wide eyes and puff your cheeks slightly during the first part of the sign to show extreme size.
2
She has the biggest house in the neighborhood.
Sign 'HOUSE' first to establish the topic, then sign 'BIGGEST'. You can exaggerate the outward motion of the first part of the sign to really emphasize just how huge the house is compared to the others.
3
Which slice of cake is the biggest?
Ask the question by signing 'CAKE', 'SLICE', and then 'BIGGEST'. Hold the final thumbs-up position slightly while furrowing your eyebrows to clearly indicate that you are asking a 'wh-' question.