It'll depend alot on the size of the packed down tent.
Down sleeping bags compact well, again, it'll be the size of the sleeping bag and sleep mat also.
Will you be bringing cooking gear? That takes up a bit of space, pots n pans etc. Stove (Trangia or something), fuel..... Water takes up a bit of space, not so much if you get a Sawyer water filter though.
On and on.........
I'm in the same position myself, but not rushing into in. Looking at an Osprey Exos 58l for example. Have the Osprey Talon 33 for day hikes and it's perfect. Don't fill it but good to have the extra space for winter clothing I reckon.
You can bring tent, bag, mat etc to a store and they should allow you to try on bags with most of the bulky kit in.