You can re-size the Visio drawing page as follows:
1. Press and hold the CTRL key on your keyboard.
2. Move the mouse pointer over the border of the drawing page, until it turns into 
.
3. Drag the border to the required size.
Note: Depending on your Visio version, you can also use Pan and Zoom in Visio window.
For publishing charts, the page size in Visio does not matter.
