For best results use 1920x1080 pixels. It could also be a multiple of that, but iMovie will create a 1920x1080 movie. This is a 16:9 aspect ratio - same as a big sreen TV.
(If you were using Ken Burns you could use 2400x1350 or 2880x1620 so you could have room to zoom in without losing resolution.)
Inches do not matter unless you are printing a photo.