Answers

So much depends on what kit you have, and how much space it takes up, smaller lighter kit has cost implications, as does less bulky food etc, if you can sterilise water then you dont need to carry your weekends supply.  Also are you able to share out tent/stove etc, group equipment.

Personally around the 50L mark suits me for a light weekend, I have done a winter weekend this way and done some serious miles and peaks with winter kit and no problems for myself,  also done summer weekend with 50L with ropes and helmets packed in too.

I use a Berghaus Arete Tour 45+8, and its plenty big enough for a two-dayer. I can get my tent and sleeping bag in, I put my Alpkit Slim Aeric folded up in the bladder compartment. The pockets on the side and lid are good for stowing lots of little bits, and the lower entry is a real plus, and it also has a zipped compartment at the bottom, great for stashing wet kit.

Efficient packing is the key, something I learned on a wild camp in the Rhinogs. I had my kipmat on the outside on day one, bit of a rush packing (the sack only arrived that morning!!), but on day two, I packed much better and everything went in no probs.

A 40 ltr sack should be easily big enough for a weekend, i use a 35ltr and except for restocking food i can go for much longer periods than that without problems.

depends on what things you have and when your going too i got away with a 45l sack in september at styhead but we had a cool snap at night and it was one of the most miserable nights i've ever had lol. you'd definitely need a bigger sack in winter

I think it all depends on how much kit you take just incase... How big is your first aid kit? Are you taking a full change of clothes, towel, emergency rations, survival bag or emergency shelter, torch, extra batteries. I could go on....

Packing to the bare minimum is fine until you find yourself benighted, or with a broken leg. Then you'll wish you had your 100L rucksack with your portable barbecue and gas TV!