I still use a very old Berghaus Dart 45L sac, for most of my walks.
Having a sac that is too large is preferable to one that is too small, and even if you find the sac is a bit too large for your current needs, at least you can use the compression straps to make the sac more comfortable.
A 45L sac can be used for weekends away, and a good capacity for a winter sac, where you know extra gear will be needed.
You do not have to use the full capacity of your sac of choice, but knowing there is extra space when you need it, is a good thing, and will result in not buying a sac that is too small, when extra purchase of kit inevitably engulfs your sac.
Somewhere between 40-45L makes a good size choice.