Vue is probably calculating the final size by the largest element in the import. So, in Poser bring in a cube from the Prop Types folder. Size it large enough for the plane to be inside it. When you export, export the cube also. So one export is V3 and the Cube, the other is the plane and the cube. So they should all come in at the right size in relation to each other. You can delete the cubes in Vue.