Just some tips for using a sling pack that work for me:
I hang my tippet spools, nippers, forceps, floatant attached to the loops on the strap, keeping them up-front and handy. I also hang up-front a small working fly box with flies I plan to use for the day. The fly box is a compartment box and I also carry some split shot, strike putty and indies in it. I rarely have to access the sling pack since everything I need to fish is carried up front.
All the bulky stuff to remains behind me and out of the way like a raincoat, extra fly boxes, spare tippet spools and leaders, headlight, water bottle, lunch, etc. I clip the second strap to keep the sling pack in place and often forget it's even back there. To me, it's almost like fishing without a pack. In fact, when I tighten the waist strap, the pack rides tight in the small of my back giving me support.
One feature I really like is I often fish early and wear a light jacket or shirt, when it warms up I take it off and have a place to store it out of the way in my sling pack. And the opposite, when I fish in the afternoon, I can stuff a shirt or light jacket in my pack and put it on when it cools down in the evening.
As far as wading, my sling pack, when properly adjusted rides higher than just about any vest. Plus the entire bottom of it is made of waterproof material so it can be dipped in the water when deep wading. You can't fish in chin deep water, so when crossing in really deep areas, I simply sling my sling pack over my shoulder and cross.
For hike-in fishing, I can carry my seven-piece rod and reel in my sling and put it together when I get to the stream. Plus I can carry everything I need for a long hike in.
As far as size, a larger pack can be left empty, ride flat and filled only when needed; while a small pack cannot carry what you need on an all day trip far from your vehicle, especially water, which you should never be short of on a trip way back in.
Finally, I really hate to change things over, with multiple packs and fly boxes. I like to have everything I need with me. I use the same pack all the time and can literally close my eyes and find anything I need. Everything I need is in my pack...I just grab my sling pack, rod outfit and wading stuff, throw it in the truck and I'm all set to hit the stream.