As I work in a big bank, I handle a lot of Powerpoint presentations. Most are cut-and-paste versions of others, slightly modified for whatever senior audience is relevant at the time.
Unfortunately the graphics capabilities of Powerpoint are limited, therefore a lot of presentations have Visio diagrams embedded in them (if I'm lucky, and not just a nasty pixelated bitmap instead).
When I'm "tweaking" a presentation, I've often found it necessary to re-organise the layout, including the Visio parts, and find that the size of the Viseo tends to go crazy, either leaving me with big white borders to crop or losing parts of the diagram off the side.
Fortunately I found the very very simple solution here:
Press the Ctrl key and move your mouse over the edge of the canvas; when you see your cursor turn into a double arrow, drag the canvas edge to the required size.
The only issue I've found is that Powerpoint can update the zoom level while you are doing this, which stops the operation and can make the canvas resize incorrectly. Just make sure you do it fast enough before Powerpoint can react!