I had the same problem and this are 2 solutions I found very useful and worked perfect to me (including a little more explanation to John SR Wlson's answer that is the best way to do it) --> I wanted my company template opened as default every time I
needed a new document, and it was in 16:9 format (this will work also if you want a blank 16:9 as default document instead of 4:3). This ain't necessary with Powerpoint 2013 because 16:9 is the default Template):
FIRST SOLUTION:
Save your design as a document so every time you open it, will have your format. The problem I had because I am a distracted guy was that every once in a while I've erased the file and have to work it again. But works.
SECOND SOLUTION (BEST!!) --> John SR Wilson's answer detailed
Once you've finished your design in 16:9 format, save it as a .potx with the name "Blank" (Save as --> "Blank.potx"). Now every time you create "New" document, it will open your design so you don't have to format it no more! (this means that if you just
want it plain and simple 16:9 with no designs, select that format and save it as told).
Powerpoint should direct you to the correct folder to save it, but just in case, in Windows 7 is C:\Users[YourUserName]\AppData\Roaming\Microsoft\Templates
If you want to reset this, just erase the file "Blank.potx" and will opened the 4:3 as default template.
Here is the link that will guide you better:
http://www.pptfaq.com/FAQ01010_Create_your_own_default_presentation.htm
Hope it will help!