You can open a 3D file or add it to an existing Photoshop file as a 3D layer. When opening a 3D file, you will need to set a height and width for the file. (3D models can be scaled to any dimension.) When adding a file as a 3D layer, the layer will use the dimensions of the existing file. A 3D layer contains the 3D model and a transparent background.
Choose File > Open.
With a document open, choose Layer > 3D Layers > New Layer From 3D File.
Choose from the several 3D model templates available.
If opening a 3D file outside an existing image, set image file height and width and click OK.
Like vector art, 3D models can be scaled up or down without losing detail or quality. Setting larger or smaller image dimensions will scale the model up or down proportionately to fill the image frame.