Just don't get one that's too big or you'll end up hearing things from your riding mates like.."could you just pop my jacket, drink, tools etc in your rucksack" and then watch them stomp you on the hills.
I have a Cloud walker PLENTY of space for a spair bike... and a 2ltr bladder although the straight bite valve has gone in favour of the 90 degree and it lacks a hose clip so i canabalised my old siren for the valve and clip