Step 1: Open your Microsoft PowerPoint Application.
Step 2: Go to the top of the application window and click over the "Design" tab.
Step 3: At the first group at the ribbon from the left you have the "Page Setup" icon, click it and there will be open a new window named "Page Setup".
Step 4: At the "Page Setup" window you have the first field (that is a drop-down list) named "Slides sized for:" click it.
Step 5: At the drop-down list you will see standard dimensions, choose one of the standard dimensions for the slide size and click the button "OK".
You can choose you choice dimension too, go to the end of the drop-down list and choose 'Custom', write your dimensions at the Width and Height fields and click the button "OK".
