It's a shame to see the "classical" sail-trees of Furaha are no longer making the reality-check cut, but the reasoning is (as always) fascinating:
- Leaf size is as much a result of humidity and temperature as it is photosynthesis (hence why we don't see giant, solar-panel like leaf "sheets";
- Leaves are good at reflecting infrared light so don't warm up too much, but they still have to cope with the fact the Terran photosynthesis begins to degrade above 26°C;
- In theory, that means warm climate plants should have smaller leaves, but that isn't true (it's inverted, actually). That's because plants have heat-regulating patterns, like lobed (wavy) edges that help and by effectively sweating. Hence, humid-region plants with wavy leaves are the best at growing big and flat;
- However, if leaves get too cold, they can freeze (particularly if they contain lots of water for "sweating") so in cold regions leaves have strategies to stay warm too, such as (you guessed it) being small and having lots of leaves instead of investing energy into less big ones;
- Leaves also have strategies for dealing with wind, including being able to curl into aerodynamic shapes, both individually and as branch-clusters;
Unfortunately, that means sail leaves would be easily broken in high winds, and could only exist in areas of high humidity with low-temperature variability. That's not impossible, but it's so unlikely (and a bit boring) to not work on Furaha.