Batteries are rated in amp hours which is a measurement of electricity and time so if I am right Ol'motorbikes battery will put out 1 amp for 3.3 hrs or 500 milliamps (1/2 amp) for 6.6 hrs and so on. So if you find the lights you want see what they are rated for you can basically pick your battery for how long you need it to last.
Of course size and price are part of the equation. I would use the same m/o as a cell phone. Pick a battery that'll last a few days and charge it almost every night. That way if you forget once it won't run out on ya'; you'll have picked the smallest battery you can afford that will do what you need!