Question: how to change aspect ratio in imovie 13
This was my question, and I found the perfect answer deep in the "other" responses by someone called macintoschforever. He gave the perfect, simple answer and his post should be brought to the top of that discussion for all the other 330+ people asking that question for the newer versions of imovie. He says if you want to keep your original 4:3 clip aspect ratio instead of the default 16:9 ratio, with your project open click on the "crop" icon above the monitor window and choose "fit". I tried it and it works perfectly. It does not crop the video to fit but brings the full frame up (which leaves black areas on both sides because the monitor window is set for wide screen, but that is just part of the deal). It is a much better solution than cropping your 4:3 movie to fit widescreen by cutting off the top and bottom of the picture.
MacBook Pro (13-inch Mid 2012), Other OS, OS X Yosemite 10.10.2