You will most likely not be able to get a much smaller sealed lead acid battery. However, you can get a battery with the same capacity as the one you currently have, but get a lithium phosphate battery. It will be physically much much smaller. Or you can run a very small battery and a capacitor. The capacitor would be used to start the bike essentially, and the battery would be for anything under 2000rpm. To determine load is something I would probably not advise you in doing, since you asked about how to determine load. The most precise way would be by finding or calculating all the wattage ratings of each individual device/light on the bike, to figure out how may amps "current" you need to be producing.