Operation Steps:
method One: Use the CTRL key and mouse
1. Open the drawing in Visio.
2. Hold down the CTRL key and place the mouse pointer over the edge of the drawing page.
3. Change the mouse pointer to a horizontal, vertical, or diagonal double arrow position the mouse pointer over which edge or corner you are depending on.
4. Click and use the CTRL key while holding down the mouse button to resize the drawing page to the size you want.
Method Two: Use the Page Setup dialog box
1. Open the drawing in Visio.
2. Click the page settings on the File menu.
3. In the Page Setup dialog box, click the Page Size tab.
4. Click a custom size, and then enter the page size you want.
The previous size is set to the default units for the base width and height (inches, millimeters, and so on). For example, if the original size is 8.5 inches x 11 inches, if you enter x 12,microsoft Visio sets the page size to 12 inches x 12 inches.
Note: For scaling the drawing page, the calculation uses the scaled size and drawing scale, and the corresponding units display size.
5. Click OK.