If its a summer 1/2 nighter, and reasonably accessible terrain, I use a 40L pack and get everything inside it except the small tent which straps up one side. However, its a bit of a squeeze if I need to take a little more gear for bad weather or in a more remote area.
I recently added a new pack to my collection specifically for this reason. Whilst looking, I spent most time around the 50 litre mark, and nearly bought a 48. However, trying lots of things on, I realised that things have moved on so much since my last larger pack (my '96 Karrimor) that going a little larger was going to add little difference to the weight. I ended up with a Gregory Paragon 58, and its unbelievable how comfortable it is compared with what I've used before. The extra space is useful on occasion, and means this pack would also do longer trips too, I just have to be disciplined not to fill it up automatically with other essentials like spare wine
It also means that when I'm walking in areas where finding water is not so simple, I can carry a bit extra without having to tie stuff to the outside, and the pack can handle the extra weight. And on the odd single night on the South Down, I can choose to lump a Helinox chair and a frying pan in to give me a comfortable night's sitting out under the stars eating a nice bit of sirloin. You can tell my priorities are not in travelling too light!
For me then, something 50-60L with a lightish construction adds a bit of versatility without weighing too much. Most of the time 50 would be more than enough. It is a bit big for normal day walking though.