Simms offers a normal size, and the large/guide size. I have the guide size and really like it. It has enough space for a full day's worth of provisions: 4 fly boxes, snacks, a small canteen, a journal, and in cold weather enough layers can be stuffed into the back pocket to keep warm all day.
I would highly recommend this Sling for anyone looking for a pack that can hold a day's worth of gear while sitting comfortably on your back, out of the way for easy casting and wading. Simms did an awesome job with this one.