My favorite pack has 24.5 liters / 1,500 cubic inches main pocket, plus side pockets and front pocket, for a total of about 33 liters / 2013 volume. I can fit 5 days worth of deluxe food rations, i.e. 3 full meals and 2 snacks per day, but if I really wanted to I could scale things down and push it to maybe 1 week of food or 6 days no problem.
I might be able to fit more after I get my 1+ season sleeping bag upgrade, which hopefully will be here today or tomorrow. My old summer bag was synthetic, and new one is down.
I should also add that I take full advantage of my side and front pockets, and also have both a chest pouch and a hip belt pocket that I use. I have never used both the chest pouch and hip pocket together, and as of late I am favoring the hip pocket because a lot of times going uphill for whatever reason the chest pouch flops around a lot and it's a bit annoying--though it is about three times as big as my hip pocket.
Don't forget water. Not sure where you will be using this pack most of the time and how much water you plan on hauling. I could max out my water load at 3.5 liters (1 liter in each side pocket in water bottles, 1 liter in a bladder in the front pocket, and half a liter bottle in my shoulder pouch), but this year I am opting for 700ml bottles in the side pockets plus a small 380ml bottle in my shoulder pouch for a little less than 1.8 liters of water total. Where I hike there is usually an abundance of good quality water to take advantage of, however.
So where are you getting the custom pack built? What fabric? What options? Any estimate on total weight? Looking forward to seeing what you settle on :)