I came to the PCT from the humid east coast, so I was pretty clueless about desert hiking and how much water I'd need. I guesstimated that I would drink 1 L per 4 miles (+ more if dry-camping or cooking). I had 6 L capacity and used it all several times in southern CA. In retrospect, I probably used about 1 L per 5-6 miles, so I could've gotten away with 5 L capacity.
But there are a couple of big caveats. First, people vary in how much water they need. A lot of folks also used Nuun tablets and the like to bolster their electrolytes and reduce water usage. I don't know if these work or not. Second, I was able to get water from all the major caches, but I started early and was ahead of the pack for most of southern CA. From the rumors I hear, there are going to be a TON of people on the trail this year (thank you, Cheryl Strayed), and one has to wonder how the water caches will hold up. In 2009, I heard from hikers behind me that some caches were empty. So, maybe be prepared for empty caches.
One tip: always cook at a water source! This makes it a lot easier to dry camp.