To answer your questions, we should know which version of Vue you use.
If you use Vue infinite or xStream, you can choose the unit system you want, meters, feet, etc...
Plants will come at the right scale, and Vue objects (vob) as well, if they were created with Vue6Infinite, and if their maker paid attention to scale. Imported objects may not come at the right scale, because of the difference between internal units between programs. Vue infinite offers an automatic scaling upon import that works in 50% of cases.
Now about terrains, yes, they come very small, if you check their size in the object property panel, they are 20x20 meters. You can either scale them up or scale down your plants.
Populationg a whole terrain that is 8x8 kilometers will indeed generate warnings about memory, ther isn't much to do about this, unless you have a 64bit system packed with a loadfull of ram.
Scale will play a role in render times if you use radiosity or global illumination, more photons for radiosity, more rays and indirect lighting shadows for GI. For large outdoors scenes, use ambient occlusion instead of GI, it will compute shadows and self shadowing within a range ( a radius), objects that are not inside this range will be ignored. Note that raytraced shadows will still be computed. Only shadows due to indirect lighting will be ignored, saving render times.
Scale is also important for radiosity, just as in Maya, the bigger the scene, the more bouncing light to compute. For outdoor scenes, choose "optimize od infinite scenery", will speed up the render.