With Prezi, aspect ratios are simple. When presenting your prezi, you will doubtlessly need to know exactly what your prezi will look like on the screen you are presenting on, Unfortunately, some projectors and screens have a 4:3 ratio, some a 16:9 ratio.
You can overcome this by setting a guide to your Prezi canvas that shows you the exact dimensions to work to for each common aspect ratio.
This is an advanced feature, and it works likes this:
When in Edit Mode, click on the options icon in the top-right corner of the screen.
Then select the ratio you wish to display your prezi in.
Alternatively, press ctrl+SHIFT+M (command+SHIFT+M on Macs) to switch between the 4:3 and 16:9 ratios or to turn the aspect ratio guide off.
Once you have selected the right aspect ratio in Edit Mode, change to Present Mode. As soon as you do this, you will see a black border around your prezi. This reveals exactly what will appear when presenting on a screen of the corresponding aspect ratio. The black edges will not be appear when presenting on a screen that has the selected aspect ratio, for example if you choose 4:3 in Edit Mode and present on a normal 4:3 projector. This is a great way to make sure that you know precisely what your audience will see. The screenshot below is based on a 4:3 ratio.
Your settings will be remembered the next time you open your prezi but can be changed at any time using the keyboard shortcut above.
Tip: In order to take advantage of this feature to its full extent, you can create frames that have a set aspect ratio. This is very easy to do, just hold the SHIFT key when you drag your cursor to create a frame. If you want to set the aspect ratio guide to 4:3 (using the keyboard shortcut ctrl+SHIFT+M (command+SHIFT+M on Macs)) when you create a new frame, hold the SHIFT key and Prezi will automatically constrain the frame size to a 4:3 ratio. The same applies if you use select the 16:9 ratio value.
Note: Prezi for iPad uses a 4:3 ratio by default, but also supports 16:9.