NOTE: This size is recommended for cameras that are 27" (68cm) or less from the rear of the camera body to the front of the lens hood.
The Storm Jacket Standard model is the most economical and easiest solution for keeping your camera clean and dry. Standard is usually the best choice if you shoot with auto-focus and rarely use a tripod or a monopod. The difference between the Pro models and the Standard models is that only the Pro models have an opening at the bottom. In all other ways they are built exactly the same.
The model or brand of SLR that you own does not make any difference when choosing the size of the cover. In fact, all Storm Jackets have the same internal diameter (8") for the front and rear openings, so the only difference between S, M, L, XL, XXL is the length of the cover from front to back. In other words, what matters when selecting a size is the length from the rear of your camera's body to the front of the lens hood when the zoom is fully-extended.