Jt33, for an August dayhike you can probably get away with a 20-liter pack, but I personally prefer one in the mid-20s. Even in August you need to carry all the ten essentials (12? 15? It keeps changing), sufficient clothing for potentially cold/wet conditions, food, and a minimum capacity of 3 liters of water once you're beyond Trail Camp. Unless you're an experienced ultra-light hiker, you'll likely be looking at a minimum of 14-15 pounds if you're properly outfitted. Some go as heavy as low-20s, others less than 10. It truly depends on the weight and bulkiness of your gear.
When you hike Gorgonio, gear up exactly as you would for the Whitney hike. That will be your best indicator of optimum pack size.
And if you don't already do so, trekking poles are highly recommended. Even if you're a beast on the uphill, those poles can be a blessing during that torturous 11-mile descent. Lots of folks coming off the trail at the Portal walk like they've spent all day in a saddle.
