How do i export movie with full size?

iMovie uses a 16x9 frame size. When you add media that is larger or smaller than 16x9 and choose fit then iMovie will size the video so it all appears in the 16x9 frame. This means you will have black bars at the top and bottom or on the sides in the viewer i.e. it's smaller. When you choose crop to fill iMovie will crop the video so that it retains its proportions but fills the 16x9 frame. It's all mathematics so if your media resolution is less/more than 16z9 you have to choose to have it smaller or cropped to fill. There are n other choices.

None that I am aware of in the MAC world. While it doesn't solve your issue, One thing you can do to mitigate the black bars is to add a white background to the timeline and then add your clip as a picture in picture. You can adjust the size and position of your clip but it doesn't change the aspect ratio.

Move to FCPX there you can set custom frame size.