Image Attachment Page – Image Size

Hi there,

Situation:

I have a Graphic Design Portfolio PAGE (not a post) with my posted work samples on it (bunch of Image thumbnails in rows – individual images, not a gallery).
The Image thumbnails are linked to Posts (link url: Post URL) via Media Library. That creates an Attachment Page for each image. If you click on the image thumbnail, it takes you to that newly created Attachment Page, where visitors can comment on the image, you can add description and so on…

Problem:

On the Attachment Page the Image is too small and you have to click it again to get to the Original size. The image’s width is restricted to 300px for some reason. I want to change that to e.g. 900px, I don’t need the original size to show up, but at least as large as my site’s width allows it.

Basically, I click my Image thumbnail and it takes me to a little bigger thumbnail, then I have to click that thumbnail again to get to the larger size. That doesn’t work for me.

Question:

How to control the size of the Image on the Attachment Page?
How to edit the Attachment Page (side bars, comment section etc.)?

The site is still local, so unfortunately I can’t post the example.

Thanks for your time!