I agree with Ron B about sizes. Those will work for most.
Shoulder straps seem to be a given. They don't have to be though. A larger torsos trap will work, coupled with a waste strap to carry the weight. Most people believe that the weight (20# or so) should be carried on the hips to maintain your balance a bit better and take the load off your spine. The shoulder straps typically carry next to no load, or shouldn't if done correctly. Your feet, legs and buttocks have much larger muscles. Anyway, this is a bit radical in concept to most…sort’a like front packs to help offset and balance back packs, very Aarn-like.
I agree that a slouching pack is uncomfortable, and, at higher weights, can cause some pain when coupled with a waste belt. Again, this is through independent research (and personal pain.) However, an incorrectly loaded pack is usually the cause of this.
Of the frameless packs I have carried, about half have had no waste belt. These were all given away or simply not real comfortable for long periods of time and not used…well, they make OK book bags. But that’s only me. I do not care for only shoulder straps while backpacking. After 4-5 hours, I like to be able to shift the weight around…without a waste belt, I do not have this luxury.
Simply placing longer objects upright in your pack will stiffen it enough to help the waste belt carry the weight without collapsing. Roll up your tarp into a 20” long, tight, roll and place it in the center of the pack, for example. Do the same for your sleeping pad and put one roll on either side, as another example. The “frame” created this way does not need to be rigid, simply firm. Coupled with firming up the pack and contents itself, you can make good use out of the waste belt, should you go that route...
To belt or not to belt…your choice. But, I will always recommend one. Worst case, you can slice it off if you find it in the way…If it is not there, you may miss it, and it will be harder to add it later.
My thoughts only . . .