Create a custom PowerPoint template – The Creative Edge

Start in Illustrator by creating a new document: File > New. I recommend starting by setting the dimensions to the default PowerPoint size of 10 x 7.5 inches or 720 x 540px, a 4:3 aspect ratio. Additionally, you should determine whether or not your client is also going to need a 16:9 template, the other standard aspect ratio for PowerPoint.

via Create a custom PowerPoint template – The Creative Edge.