All I was going to say was usually I take a 45L/50L pack, but if it is just Summer only for a night, there's no hobbies other than drinking involved and I can do a hammock, I'll take a 30L. Winter, and if there's snowshoes and that which may have to be carted, with a tent, sleepmat, loads of bulkier spare clothes etc, I can add a 10L toplid to the 45L ruck I have and maybe get away with it .. Or, take the 70L one. I don't think I have ever filled that one up.
Usually, its the 45L one though. Summer means light sleeping bag. Bivvy and tent need a sleepmat, hammock doesn't (in Summer). That's the sort of equation you want to make. The bulky stuff. Even squished right down, my winter sleeping bag is huge compared to my summer one.
Other stuff doesn't change ... cooking gear, food/water, 10 essentials bag, a little first aid bag, spare socks, room for a waterproof, woolie and hat etc. Those basic things are pretty constant.
As for brands, I don't have one of these but people seem to really appreciate Savotta at the moment. Gregory, Deuter, Osprey are all good, light, tough enough and affordable. I like older Kifarus. Heavy and expensive, but very comfortable. Then there's the Karrimors. It's all good.