Soilless mixes are good at wicking up moisture. That moisture evens out throughout the mixture just like water vapor in a room. There is a certain carrying capacity beyond which they no longer wick up the moisture. New mixes tend to work better for this. The traditional tradeoff in a wicked bottom watered situation is that the soil in the water is overwet, leading to the death of roots in that area and the growth of undesireable elements. The Earthbox design limits this by limited the amount of soil which is actually in the water.
The only way to adjust the moisture in the soil is to adjust the mix itself. Fortunately, a good light soilless mix wicks about the right amount of moisture for most plants. If they use more water, more is wicked up, and vice versa. Bottom watering also results in less compaction, so aeration is often better.
Beyond a certain height, the evenness of the moisture in the mix drops off and the concerns you expressed become more important. Many plants, and most of our water-loving tropical garden plants don't actually require an extensive root system to thrive as long as their basic needs are met, so they do quite well in an optimized environment (water, air, and nutrients in the soil) even when they have less space.
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