A relatively large print size was chosen on screen but came out much smaller on paper
Would you mind to explain how you “chose a print size on screen”?
The question is really too vague to answer… but looking into it I understand there are some troublesome effects when viewing a page in 100%.
You will have to change the zoom level in Writer (double click on the [digit]% in the lower right corner), so that when you hold up a paper over the screen, the paper will be the same size as the page behind it. Or you could show the ruler in the View menu (View / Rulers / Rulers), to see the actual size of things, in centimetres or inches.
When I set the manual zoom level to 135% the ruler marks lines up with the ones of a real ruler.
The LibreOffice Writer manual says 100% zoom level is the “actual” size, but there must be more to it, but I don’t understand what setting I may have that could be the cause. My display is set to 100%, but 125% is said to be “default” for my screen… I’m running Windows 7 for operating system.
There is a scaling setting for the user interface under Tools / Options / LibreOffice / View. Setting it to 135% the page gets the right size at 100% LibreOffice zoom level (same as print size), but the whole user interface is enlarged, with menu items and all…