[eluser]Crimp[/eluser]
It seems you are confusing the issues.
The image manipulation class only manipulates the image.
You want to check if the image, uploaded or already on the server, is a certain size and based on that size, it is either manipulated, resized, or not.
That's two different tasks.
First, determine the dimensions of your image using either the upload data array in the CI upload class or a PHP function, getimagesize().
Now you have the width and height of the image. Then write a conditional that sends it to the resize function or not, if < 400 x 600.