According to the DK documentary Eyewitness -- Life (VHS), Galileo correctly calculated that the tallest Earthling lifeforms can get is ~300'. The tallest Sequoia's are almost exactly that tall. I'm told this has to do with the limits of Capillary Action, which allows organisms to pump water up to that height. PARALLEL: According to the National Geographic documentary Naked Science -- Earth's Crust (TV), the tallest possible mountain on Earth is only 45,000'. Any taller, and Earth's gravity creates such pressures beneath its base, that the crust melts, and the mountain sinks back down. But, on Mars, whose gravity only around 1/3 that of Earth, Olympus Mons is 90,000' tall, 3x that of Mt. Everest. CONCLUSION: Earth-like lifeforms, limited by gravity, could grow taller on smaller worlds, but would be shorter on larger worlds. On Mars, w/ ~1/3g, Seqoia's could grow to nearly 900'. But, on a 2.0g planet, the tallest "tree-forms" could only grow to ~150'.