How tall are fully assembled trees? Can they be scaled up to about 6” high and still look good?

Adding leaves do not add any significant additional height to the assembly. Using the numbers I added to the sketches will give you the best idea. You can remold the leaves by using the warm-water method for creating different shapes too.

You can definitely scale these models, however, the base will scale as well. There shouldn't be any resolution lost if they're scaled.