1. Go to "Layout" click "Edit HTML"
2. Tick on "Expand Widget Templates"
3. Find </head> code
You can cutomize this size
summary_noimg = 430; is post cut height without image
summary_img = 340; is post cut height with image
img_thumb_height = 100; is thumbnail image height
img_thumb_width = 120; is thumbnail image width
5. Find this code
6. Replace with this code below
Now you can try to post and it cut automatically.
or download readmore-otomatis-script here
Source : Kang Jaloee Punya