The new Hero4 cameras, and the Hero 3+ Black, feature 1/2.3-inch type 4:3 sensors with 4,000 x 3,000 pixels. This is physically smaller than what you'll see on many other digital cameras, but it's very much the standard for actioncams (where you don't want the much bigger lenses that bigger sensors require).
GoPro has not revealed what size sensor is used in the Hero 3+ Silver, but we do know that the original Hero3 Silver used a considerably smaller 1/2.7-inch type sensor.