Wouldn't you love to sit all day on the steps of a bridge and watch all the canal activity below? Don't plan on it because foot traffic is often so very heavy on most bridges that signs are posted to tell you "No Sitting on the Bridge". Also, keep to the right on a bridge or you might get mowed under by oncoming walkers. But such is Venice where bridges are a key way of life and unbeatable places to quickly snap wonderful photos of gondolas gliding below.
So your camera can't manage to fit all of St. Mark's Basilica into a single shot? Don't worry - many cameras can't. Just find your favorite portions of the exterior and shoot them as close up as your lens will allow. Shoot enough images and you'll surely have some fine photographic remembrances of your visit there. Besides, you'll enjoy seeing the greater detail on your various closer shots. Why you'll have your own art gallery of magnificent stone work, Biblical illustrations, allegorical works, and even historic horses (see my website photos of these special steeds) .