Using [CTRL] + [ALT] + Frame could result in a frame with a border ratio of 4:3, because most projectors use this.
[Shift] + [ALT] + Frame maybe could be 16:10.
This could avoid some pitfalls.
Haha, found the shortcut: [SHIFT] + Frame.
Should be better propagating within the help. I didn’t found it there.
Adrian,
Which ratio does your shortcut render? Thanks.
The aspect ratio between width and high of the monitor/projector. It was typical for televisions, monitors … before the wide screen era (16:10 at monitors and 16:9 at televisions). Video projectors mostly still only support 4:3.
I think for presentations (especially in science) it is better to use this ratio for your “slides”. Avoids anoying effects with neighbored slides.
Ah, [Shift] + Frame makes 4:3.
This has been an issue for me for a while. My laptop is a wide screen, while the projectors in the classrooms use 4:3 and I repeatedly get things wrong when creating “slides.”
Unfortunately, I can’t seem to replicate your trick for creating a 4:3 frame. As a scientist, I hope that you can have pity on a poor, dumb teacher in the arts and tell me your step-by-step. 
Thanks.
Figured it out!!!
Click on “Frame”
Then hold “Shift” while drawing frame on canvas
Result is that Prezi will only draw a 4:3 frame
GREAT!!! I’ll be using this a lot. Thanks.
You got it