when you explode (split) a .3ds file mesh in VUE, depending on what exactly you exported, you will get what you say, each mesh on its own layer, dependant on your export options from your 3D modelling program.

I don't think there's a way to globally set the plant objects in VUE, that's why I say, assume Vue units as a certain measure, then, when you import a plant, you can type in it's Z dim. so you know how tall it is, then, knowing what it came in for a Z at default, you can know what ratio you had to scale it to get it to be in feet, etc...

I highly suggest a hybrid of Wabe's and My methods for scaling in VUE.  I hadn't thought of Wabe's before, I wonder if you could just have the 8 points of a corner of a box that surrounds each of your objects, but either way you can just delete it later in VUE.

again, though, VUE really isn't ready for Architects yet.  It will turn your 8 hour project into a 2 week project.