The video tutorial of this topic can be found here.
To scale, use the pinching gesture to increase and decrease the size of your 3D model. The scale of the model will appear in the text box at the top of the screen. Scaling may not be available for all 3D models as it must be enabled by the owner.
To return the 3D model to the scale of 1.0x, tap on the (Scale + ⟲) message.
Learn the other gestures for manipulating your 3D model here.
To learn how to freeze the scale of your model, or to set initial scale, check here.