i hiked with a children's school backpack for the first 900 miles or so. It was fine, but i couldn't get hitches. Anyways, it's just like any other hiking--it's pretty much all up to personal preference. You will need to carry a max of 5-8L of water (or carry a few liters less and do significant night hiking, take your pick)
I didn't use a canister and if (err when) I do the PCT again I still wouldn't use a canister. My pack thru the sierrias obviously couldn't fit one, and none of the rangers i encountered seemed to care one bit. YMMV