Image size Problem

Dear team, I am using Full Text RSS version, I am grabing content from other site and posted in my site with images, now when content is posted the image size is too large and all theme are out of order, so I manuall go to the post and select the image and change the width from 700 to 600, Now can i do this by default, so when ever any image is posted in my site it automatically set the width to 600. ??
Like this is my site example

http://megacitystore.com/?p=32