I want to print some images to be exactly 7.9X12cm, I want to print multiple images a page because its a waste of paper to print one per page. I am trying to import all the images onto a single krita document so i can print that( If i try to select the images individually to print on one page, the printer software auto adjusts there size and I cant customize it through the print settings). I have the images set to be exactly the size i want but when i paste them into one document they change size at random.

Is there a way to make sure the images retain there size after copying and pasting, or change the size of an image but not change the whole document size, i cant just use the transform tool, i need to know how big i am making each image. i tried to use the 'scale image' tool but even if i just select the image i want the whole document still changes size.

The thing to remember is that krita doesn't know about printing and doesn't have a clue about your printing software or how your printer behaves. All it knows is how many pixels high and wide your images are.
As such, it's up to you to manipulate and arrange the image so that it prints at the size you want with your printing software and your printer. You'll need to do some simple calculations when you're doing that.

Where did you get the images from?
How do you know they're exactly the size you want?
What is their size in pixels?

If you paste two different images into a krita document and they're different sizes on the screen then it's because they're different sizes, as measured in pixels. Pixels is all that matters in krita and similar digital image software.

If you use Image -> Scale Image to New Size then yes, it does operate on the entire image.

There is a fairly easy way to ensure that an image gets printed at the size you want and that is to make a template document for your printer. With rectangular guide frames arranged and sized so they print out at the exact size you want.

Then you can adjust your imported images to fit inside the frames. You put the frames on a separate layer so you can turn them off before Export to .png for printing.
First you need to establish the size of the largest rectangle that can be printed because it's unlikely that your printer can print all the way to the edge of the paper. Then keep the guide frames inside that large rectangle.