I spend three months in China in 1997 (went for a month in 1996, spend the year learning conversational Mandarin, and went back).
If you are not going to bring a tent and sleeping bag, cooking kit, etc etc than I think you can stay really small.
If I were to go again I would try to take a 32L panel loader, perhaps a 38L which would be more than enough for me.
I had a lot of gear with me (Chinese language books and lots of rolls of slide film included) and used nothing except two t-shirts, long and short pants, couple of shorts/socks and a sweater. Remember you can also get gear in country.
I traveled on bus/hitch-hiked,road horses through the mid to west to south in the second trip and the only thing that kept me busy when I arrived in a new village was getting water/food and a place to stay for the night. Then how to get transport later on to continue the trip, then perhaps a shower/dump in the lake/river. Really would have liked a sleeping bag liner when sleeping on concrete floors in somebodies storage room with heavy wool blankets on top of me.
I would just get gear that might be difficult to get there, like your favourite fleece, UL down jacket or rain gear.
One point about packs.
They went on top of cars or carts or busses, or in the bottom. There is almost always flooding, and if your pack is in the bottom, it will get wet from below. If it is on the top it will get wet from the top. I had a pack cover, put it cover down when in the hold if busses would wade through flooded areas. If your pack is smaller you can take it with you inside the car/bus. Lot easier.
If you want to take a lot of gear, you can also think about a bigger semi waterproof duffle (like a North Face Basecamp Duffle) and leave that where you stay, and have a smaller day pack. Personally I would just take one bit bigger day pack for everything, but it depends on your intensions too (staying longer in one place all the time vs mostly moving).
If you have time, would be nice to learn the language. Lot more fun if you can talk to old men/women you meet on the trail, and gives you more freedom to arrange transport or go places there is no tourist transport for.
Sounds like a great trip – have fun.