Instagram HD with imovie

Hello, world.

/ i have some sort clips take with iphone 6 camera. (not with instagram app)

/ and if I'm trying to upload to instagram they crop/chop the clip so the clip is not usable.

/ i m seeing other users have HD good quality clips on Instagram

/ so the question is how can i resize the clips i have., with imovie in mac OS X yosemite.

/ i don't like to buy other apps or use anything that request money.

i need to know the instagram size accepted , and how to resize , how to place that black/white bands in imovie.

thanks a lot.