You may need to adapt the embedded drawing to the horizontal orientation of a slide—to resize it, that is, so it's wider and shorter. Advice: Do this in the Visio window that's adapted to PowerPoint (double-click the drawing to open it this way), as this view can help you by keeping the slide and its size in view.
Steps to resize an embedded drawing
First enlarge the drawing page by dragging the side sizing handles of the Visio window.
Then, select the drawing itself, and drag the corner or side sizing handles to make it bigger, as shown in the picture. This may flatten the look of the shapes. Enlarge them, and the font, if necessary, in Visio. You'll do all this in the practice session.
Click outside the Visio window to close it and see the drawing on your slide.
Linked drawings If you need to resize a linked drawing to fit the slide's orientation, it's better to do this to the original before you link it. If you resize it in Visio, opened from PowerPoint, sometimes the changes cause the whole drawing to be smaller in PowerPoint. Another option is to resize it just on the PowerPoint slide without opening it in Visio.