I have some contrary advice. Don't get hung up on getting a green bag! By which I mean, don't get fixated on the Sabre and similar packs and ignore the stuff in outdoor camping/hiking shops. Military packs look the part, and are built to be durable, but they often have other features or characteristics that are not so great. I have used a the following packs:
- Snugpak Rocket pack,
- Sabre 45,
- PLCE side pockets + yoke,
- Sabre 60-100 (early version),
- Kifaru Zulu Mk1
- OMM Villain Mk1
- OMM Jirishanka
- Kifaru Tailgunner Mk1
I got rid of the Snugpak fast, the back was so short that it just about did me in, was happy enough to return the 60-100 to its owner, sold the Sabre 45 when I realised it was hardly used, and passed on the Jirishanca. The Jirishanca is the only one I am a little sorry about!
Both OMMs came to me cheap because they had straps that needed re-sewing, so I can't say that they are anything like as tough as the Sabres, but as far as nice features go, they blow the Sabres out of the water. The 45 really wasn't all that comfortable to carry, and I have done a lot better with the Zulu than I ever did with the 60-100. You really don't need 1000d Cordura most of the time! Certainly don't need IR suppressive 1000D

Don't need packs designed to work in conjunction with webbing and belt kits (Snugpak). You may not have the body shape of the typical soldier, so packs designed to for fit 20yo may not fit you so well.
Fit is REALLY important! More important if you are not used to carrying loads on your back. Much easier to try the fit of packs in camping shops than trying to find somewhere to try out the military style packs.
While we would all like to think that we are going to be hacking off through remote wilderness, in reality most of us don't get out for more than a week of light camping at a time and don't get the full benefit of tough, heavy gear.
45litre is enough 3-4 days but only for people who have their packing and kit all refined. 60 litres will give you more space to figure things out. You will undoubtedly pack more than you need, but the best place to learn that is while you are out, much better than not knowing what to fit into a bag that is small, and packing the wrong things.
Sorry for the ramble! Best of luck