I am new to Sketchup. My first model is 28mm x 20mm x 9mm. How do I set the initial scale (namely zoom in) ?
Zooming in is normally done with the scroll wheel or the Zoom tool. You could also choose Zoom Extents.
FWIW, it’s not really “scaling”. You do your modeling normally at full size. You’re just making adjustments to the camera so you can more clearly see the model.
If you are trying to set your view (and you can save it as a “scene”). You can draw your box then orbit and move the camera to suit yourself.
Until you draw something, you can’t tell what the scale is in the SketchUp view window. So, draw something the desired size and then do zoom extents so it almost fills the view. As noted, you draw at real size (or sometimes at a larger size if small geometry is involved), not at a scaled size.