And the 50-500 is about the size of a football... The "kit" lens is probably a good starting point, and should be about 18-70mm. It won't be anywhere near as long on the telephoto end, but you could also add a 70-300mm lens.

The equivalent focal length in mm is compared against 35mm. With a Rebel XT you would multiply the lenses you would use on that camera by 1.6 to get the 35mm equivalent for that camera, because the sensor is that much smaller than 35mm film.