My wife and I plan to hike the JMT around that time frame; small world, maybe we'll see you out there.
I'll likely be using a
ULA Circuit to do this. Last time I went through the JMT, I found that folks doing that stretch all seemed to have relatively large packs, and given the need to carry a bear cannister, the Circuit will likely be on the tight side for me. Hmm, in fact I had to carry just a "weekended" size cannister on a trip last year in that pack and it was on the tight side, so you might consider the Circuit as more towards the lower end of what you might see on that trail at 4200 c.i. total, but only 2400 c.i. of that capacity in the main pack body. One ends up strapping relatively more stuff on the outside to push the limits of what can be carried.
I'm not an "ultralight" backpacker, but wow --- the actual "
JMT thru-hikers" I saw on trail in June of 2008 often seemed equipped for an arctic expedition or something.
In any event, given that you don't have a lot of experience at this stuff, I'd definitely suggest a larger capacity pack for you, for this trip. It takes time, experience, and typically some money to whittle down to a smaller volume of "stuff" to get into the smaller volume packs.
Do consider whether you anticipate strapping a bear can to the outside of the pack or fitting it inside the main body --- that will make a really big difference in required pack capacity. If you go for external strapping, think through this carefully as a heavy, fully loaded and smooth plastic bear can wants to shift all over the place on you. Using a different pack last time through there (a Gossamer Gear Mariposa Plus) I started into the Sierras with the can strapped to the top but found that to be a real PITA, and so shifted things to barely scrape by with the can in the main body.
FWIW, here's a
picture of me with the can still on top, before I wised up and moved things around.