I am not sure with LED fixtures. Do they have the same amount of bulbs per length? For example, does the 48 in fixture have 2x as many bulbs as the 24 in? If so, it shouldn't matter.
With florescent lights, it's a bit easier. A tube 2x as long with double the wattage would put out the same amount of light, more or less, than two tubes half the length. I imagine it would be the same with LED's, assuming you have the same total number of bulbs of the same wattage, in the same places.