You can simply place any object in the world using World Object Controller.

To get the World Object Controller, simply go to the Asset Library and search for World Object Controller and then click on the import button.

TIP: You can find the World Object Controller under the Tools section.

Now drag and drop the downloaded World Object Controller prefab from the Resources panel, into the Objects panel.

As you can see the objects is shown really huge on the screen.

To fix this we need to add Device Tracking component to our camera so our camera can follow the users movement and stick our object into the world.

In order to do that, select the Camera object and then in the inspector panel click on Add Component, and search for Device Tracking and select it.

Now you can see our character is now not huge anymore as well as stick to the world.

At this moment we just need to set a reference from our camera with device tracking to the world object controller. To do so, click on the World Object Controller in the Objects panel, then in the inspector panel set the camera to the Device Tracking input as shown in the gif below

Great! Now everything is setup, we can change the content to whatever that we want.

To do that expand the World Object Controller in the objects panel by clicking on the small arrow.

Now delete the Red_Panda_FBX object and place your object as a child of the World Object Controller.

Now feel free to set the size and place your content inside the World Object Controller.