70GI is the standard for track bikes on the road, and its a nice comprimise for accleration and hills, but still enough length to be able sprint at speed on the flats. witht his said its more important to choose a gear that fits your level of fitness/strength and the terrain you ride.
i ride 70", but i live in an area wiht undulating terrain, if i lived in a flat city id probably go up to 80".
i recommend getting yourself an 19,18 and 17t sprocket. try the 19 out for a couple of weeks even if it feels a bit spinny. then try the others and see how you feel. remember its better to spin than mash, and riding in a gear thats too high is only going to give you big but ineffcent muscles and maki your average speed low.