is there any way to change the post editor size ? i want to get the same size like when the post is published
Down in the right corner of the post editor, you can drag and drop it bigger or smaller in the way you want it to be sized.
Agree with misel's answer above, however you can check, in Settings -> Writing -> size of the post box -> and change to 30 or 40 lines. It is by default 20 lines. That way, whenever you create a post, the box will be bigger than usual without dragging the box as an alternative.
when the post is published the size is set on horizontal(600px) not vertical so i want to have the same size(600px) on the editor so i have more clue of how the post will look