700 pixels is the maximum. In your editor, go to resize image, you should see the X and Y axis size. in the larger of the two (usually the X axis) put in 700 (or less). Most editors default to uniform scaling so it will auto shrink the other axis in the same proportion. Then save as (don’t overwrite your original file just in case you mess up) and put it out on your desktop. Then you should be able to attach it to a post.