How can i increase the size of color box pop up window in drupal 7. .?
You should probably check the settings at

admin/config/media/colorbox


If the issue is that the image is too small then you need to change the Image style to a different one or perhaps make a new one.

If you want to make the Colorbox larger you can use the Custom options on that same configuration page and set your own defaults for size. The Colorbox module should allow you to specify


initial width
initial height
max width
max height


If you choose Custom under the Options setting.