I am 5'11" and I use a Keepall 45 regularly as a daily bag. A 50 would look like luggage with my height, this can still pass as a big hangbag. I feel that the Speedy 40 is somewhere there too for someone your height.
You can carry whatever your heart desires! I say you rock it! I am only 5'2'' and I carry a speedy 35. I know its a little too big for my body frame but it makes my heart sing and that is all that matters. GL!