I have an Arcteryx Tango which is 65 L (got it the the North Van Arcteryx Store for ~$200 during one of their outlet sales). For me, it is perfect, I'll be going this time to South East and East Asia this January/Feb for just under a month. Packing in 7 days worth of clothes and i have some room if I want to pick up new items along the way.
I definitely recommend a tote/cover for your backpack as well (just like what @T4RAWR mentioned):
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