Why do aquatic plants such as phytoplankton tend to be very small whereas most terrestrial plants such as trees tend to be larger and have more specialized structures?

Consider the challenges that land plants have to face:

1. The struggle for light. If a plant doesn't grow tall, it will get shaded to death. In the sea, growing tall just means sinking, so there is no advantage for an aquatic plant to grow a tall body.
2. Defense against dessication. If a plant dries up it will die, so it is advantageous to have water storage organs. In the sea there is no water shortage, so there is no advantage for an aquatic plant to grow water storage.
3. The search for nourishment. If a plant's roots cannot find enough nutrients, it will sicken and die. In the sea, the nutriants come to the plant, so there is no advantage in growing roots.
4. Fertilization and distribution of seeds. If a plant can't get its pollen to a mate's eggs, then it will not produce offspring and will become extinct. In the sea, the water will take the 'pollen' and 'seeds' where ever they needs to go.
5. Defense against herbation. If a plant gets eaten, it will die, so it pays to grow bark and thorns. In the sea, growing a thorn big enough to deter a whale just isn't feasable.

In the sea, size is not an advantage. It pays to be small and simple. The one exception is kelp, which grows large and complex - mainly to keep the 'plant' in a favorable location.