Hi Alan
I am 6 foot 3 inches so not quite your height and have just about given up on back length - and on almost any sac the hip belt almost never sits on my hips so does not assist with taking the weight, just stability. But I use a long Osprey Tallon 33 which I like very much for day walks in winter and summer - it has a pretty generous capacity for 33 litres, is reasonably light, comfortable and I have "grown" into it. For larger loads the Golite Pinnacle, long length is superb - and it compacts down really well so can be used for just a day if you want - and it is really light.
David