Resize Images in a post

Hi, I am uploading images generally as largest 640 or 800 max size.

When creating a post, and inserting an image, I may want to resize it, so in one post, 1 image is smaller than the other.

When I insert the image into the post, I go into that image to edit, and change to say, 60%. It then resizes in the post I am editing. However, when I preview it, it comes out as the full size again.

Is there an easy way to create a post, but have all the images as different sizes without uploading them as the different sizes in the first place?

Any help appreciated
Many thanks
Richard