A pack of around 65L should be fine for a week when carrying all food with you, depends most on the size of your sleeping system.
I started with an 88L Highlander Forces which I now use for bulkier trips (got more compact gear and better selection of gear over the years) and now use a 50L Lightwave Fastpack whenever I can. I'm looking to buy a 30L pack to play around with going as light as comfortably possible now, quite addictive
A weight of around 15kg should be no problem if you can refill your water reserve on the way (springs are often indicated on maps). I usually carry 2-3L of water after a fresh refill, depending on how far the next spring is. I count with about 0,5kg food/day, try to keep it tasty and healthy while going for high energy density food. On longer trips the food becomes a big part of the load but I find it quite a challenge to optimize the food without buying expensive specialist pre packed meals... I might need to start cooking properly at home to learn more about preparing food myself
Don't buy a pack that's way too big, it probably won't be very comfy when it's only half full. Better buy a bit smaller, makes you think more about what you really need and will probably end up comfier on your back.
I'd advise against buying a very expensive pack the first time, it's very likely that you'll be buying an other pack that suits your use better in a couple of years when you know better what you really want/need on a trip...
Whatever you do, have fun!