Changing post width for bigger images

Hello.

I want to have my images on the single posts appear bigger.

(site: )

On the computer my images (not the featured images) on the single posts appear
small and because of that blurry (I use MS Paint a lot and resizing in that is difficult to impossible).

My images are often as wide as 2000 px and i’d like my images
to appear as big as possible on the single posts.

i now used this bit of css via a different topic:

body.single .entry-author, body.single .entry-content, body.single .entry-footer, body.single .entry-header {
max-width: 980px;
width: 100%;
}
.wp-caption.caption-big, .image-big {
margin-left: 0px;
max-width: 980px;
width: 100%;

I put the max width to 980 which does make the images appear bigger
but i want to set it higher. Preferably i want to set the max-width to 1000 or more but when i do the single pages get wider and a scrollbar appears. Not very neat.
also, in effect, the grey overlay box and title are now not centered.

If anyone knows how i can set the max width higher, and center
the overlay box and title, i’d love to hear it.

Thanks for all the work!