image size in wordpress gallery, please help.

I want to add image gallery in my post. The problem is that when i click on the image, it opens the “?attachment_id=x” page, where it displays the image of with dimensions approx. 300px × 189px.
I want to display the original size of image, not the resized.. i can not find any option to do that.. please let me know how can i make it.
thank you so much.