A lot of that question depends on the time of year you will be hunting. Early season you can get by with an 1800. The further into the year you go you might want to increase your bag size to accommodate some extra clothing. Or you can stay small and pack your extras between your pack and the frame. My style of hunting I have two packs both around the 2000 ci range. I day hunt and I carry an extra coat for when I am stopped for extended times glassing along with my butt pad.
If I've packed in I use my Nomad and a #11 pocket, so i'm guessing 2700 cubic inches. That carries everything pretty well. If it's a day hunt kind of thing, then most of my packs are 2800 to 3000 ci. If I was hard pressed, I could get by with 1000 ci. For my back, I like to have a waist belt even on the little packs. I'm pretty fond of the Kifaru Zippy (even though it doesn't have a belt), and tailgunner. I've used Kelty Redwing, REI, no name brands and they all got the job done. Some wore out faster than others though.
I think the Kifaru Spike camp is a perfect day pack. I am not a minimalist though. I also have an EXo 3500. I only need one and am selling my Spike camp if you are looking for a pack.
my smallest is the Sitka Ascent 12 (1,200 cubic inch) but mostly use it for archery season (end august/sept) where i dont carry as many layers. It tends to be pretty full and earns you a return trip to camp to get a frame pack without carrying much back with you. It is 4lbs lighter than my Kifaru though.
I use a USMC ILBE "assault" pack, designed by Arc'Teryx...it is bomber construction and pretty comfy. Not the roomiest for long days but you can strap a lot of stuff to it. Bought it on Ebay for like $12 bucks, they're going for a lot more now...
Badlands Reactor for me. Only 900cui, but I can get everything I need in there for a guaranteed warm/nice weather day hunt (water, snacks, phone, wetwipes, ammo/release etc)
I use a Badlands Superday pack, the older version not the newer version. Had it dor years and can't get my self to change it. To me it looks like the version has less pockets and compartments but I could be wrong.
I've been using my Cabela's Elite Scout pack (1800ci) since 2005. I can't believe that it's still in one piece let alone even functional. Tough quiet little pack that has served me well. I hardly even notice that I'm wearing it. It weighs 15lbs with everything I need for the day, kill kit, and water.
I use my KU5200. It cinches down as small as any daypack, but carries a load out that no day pack could. Weighs less than just about any daypack too. Being able to carry that first load out right away is huge.