Not sure what iMovie you're using, i use iMovie HD, GoPro on 720p, 60fps.
When exporting i go
expert settings -> share -> options -> settings
In settings choose compression type h.264, then under data rate, click restrict to....then type 5000
exit settings, back to options and click size then HD 1280 x 720, 16:9 and also tick the box 'deinterlace'
that gives decent HD quality at a small enough file size to upload to vimeo/newschoolers etc.
hope that helps, difficult to give instructions without visuals
if not, just youtube imovie HD exporting, thats how i figured it out.