I'm looking for a backpack for a one day Whitney hike in August 2016. Not sure how many liters. I'm thinking about either 26l or 34l

Looking for opinions please

Gear list

Food (not sure how much)
Gortex jacket
First aid kit
3 liter water reservoir.
Extra socks
Extra shirts
Roll of toliet paper
Trekking poles (tied on outside)
Any other items that I might need are welcome

Thank You
Brian