A device, system and method for calculating a size of an object using images acquired by a typically moving imager, for example in the gastrointestinal (GI) tract. A distance traveled by the moving imager during image capture may be determined, and spatial coordinates of image pixels may be calculated using the distance. The size of the object may be determined, for example, from the spatial coordinates. The moving imager may be in a swallowable capsule, or, for example, an endoscope.