Unit sizes aren't changing.
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It would be far too much work to rebalance everything. Projectile speed and range, hitsphere size, unit speeds, turnrate... there are a lot of variables and many are non-linear.
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Units are about as small as they can go, Flea has a 1x1 footprint. Spring likes to work on the scale we have.
If you want 'smaller units' play on larger maps. There are plenty around but they are not popular.
Planes already fly high enough to not be hit by most ground. Gunships are suppose to fly that low so they interact with ground units.