In most cases the target images will be printed. The resolution for printing is generally higher than video.
A Full HD video (1920 x 1080) at print resolution equates approximately to an image of about 16 x 8.5 cm, so you can generate a target from printing a frame from a video clip.
In order for the image to be recognizable easily by the camera, it must be taken into account if the user can handle the target, that is, if it is printed on a leaflet, card, packaging, etc. In these cases the target does not have to be very large, since the user can approach the target of his camera. A minimum of size is required so that the device's camera does not have to be so close that it can not focus on the image.
An image of the size of a standard business card (9x5cm), for example, is enough for good recognition.
In case printed targets in non-handy locations such as banners, billboards, etc., it is critical that the target image be large enough so that the camera can recognize the image from a distance.