If it is for an octo than a longer tank would probably be preferred. However, if u did decide to go with a taller tank than lighting shouldnt be much of a problem seeing a octos dont neccesarily need lights (and if they do they certainly dont need to be bright lights).
Hi Jesse, personally I would luv to hv a tank with 8ft in length and at least 2ft wide. Height wise I would keep it to 2ft, max 2.5ft. So long my arm can reach the bottom of the tank, cleaning and moving rocks and corals around won't be too much of a chore.
I guess the limitation of length is up to your space. Of coz the filtration must be sized right, which with enuf $$ can always be done right :P