Again, do not be fooled by seeing Eurasian backpackers backpacking with their bags on their bag.
When you meant backpacking trip, do you intend to bring your bag everywhere with you whenever you travel? How are you travelling?
If you're not travelling much and doesn't need to bring your bag everywhere you go, why would you need a backpack? Wouldn't a bag with trolley wheels or spinners even easier to bring around?

Personally if I'm going on holiday, I don't really like to bring backpack. One thing I don't like is packing of cloths. With a normal luggage, I can just unzip and find my things easily instead of stuffing my things into backpacks and sometimes when needed, have to take everything out because it's in the bottom of the pack.

So think carefully hat's your purpose and why you need a backpack before buying one.

If that is the case, then get a Deuter. Deuter got "SL" range meant more for woman.

16 days is very long. You'll definitely need to find laundry shops.
But anyway, you can also look for "compression sack" something like this: http://www.seatosummit.com/products/display/39
So you pack your cloths inside and there's 3-4 draw cords to let you tighten it i.e. to compress your clothing.
How big a pack you need actually depends on individual. Try prepare a list of items you want to bring. Then go shopping and try to imagine to see whether or not it will fit.

Backpacks with trolley, yes there is. I think TearProof got. But whether or not it's comfortable I'm not sure.