I want bigger images in the gallery

After clicking in a photo in the gallery I get my images with the width of 300, but I want at least 600. I know that the code generates this image but I don't know where to modify it.
Does someone know how can I get that?

Hi!
To fix this you have to upload image and set "ATTACHMENT DISPLAY SETTINGS" --> size on the right before adding this image to your post/page.