I found an article about how to expand the root size from Cooperative Linux Wiki. But I am wondering if there is any safe ways to reduce the root size. Thank you!
I have found the solution by using tar and creating a new file system image.
http://colinux.wikia.com/wiki/BuildingColinux0.8.0 , section "6. Create, resize and optimize images" is the proper way to reduce the image size best as can.