For multi-day pack I like between 60 to 70L packs. I'm admittedly not the most efficient go-light hiker but this size serves me well. After all, you don't have to fill a bag to capacity! The larger bags tend to have better internal frames (at a weight sacrifice), really good padding, wicking mesh around the back and extra thick waist straps.
For a single day/weekend I like a 40 L pack with an internal frame (lighter than the heavy bag). I find that I end up taking the same amount of equipment for a two day stay as a multiday one - except for less food. If I'm at the top end of capacity of the 40L bag - I switch to my 60 for comfort as above.
For day hikes it all depends on what I'm doing for food and if I'm bringing equipment along. e.g. cooking a meal requires more gear than just going with sandwiches and water. Often a typical school bag is good enough for these purposes - unless I want to bring an axe and other tools along. In that case I would go with the 40 L pack and internal frame for comfort.
Dr. Thor - you might find the 40L a good all around bag for over-nighter/multi-day if you go ultra-light on the multi-day. I still think that you will find it a bit annoying for a light day pack. Maybe a good quality 40 L and a $8 wallmart special book bag would be good.