One of my students has a term paper. She is using Times New Roman font 12 pt. When she goes to print her paper it prints the bibliography in 9 or 10 pt. font even though it shows on the screen ad 12 pt. In order to get it to print to the right size I had to select all and change the font to 18pt.
What OS and what version of LibreOffice are you using?
No answer but I am having the same problem with LibreOffice Calc. I set the font in a spreadsheet to 14 pt. which looks fine on the screen but when the document is printed the font looks very small ( about 10 pt.). This occurs with different printers. I am using Ubuntu 12.04
and LibreOffice 3.5.4.2.
Hmm. Looks like we might need to get down to brass tacks on this one! 
From what I gather from wikipedia, the point is ≈1/72nd of an inch. Furthermore,
"the height of an em-square, an invisible box which is typically
a bit larger than the distance from the tallest ascender to the
lowest descender, is scaled to equal the specified size."
Wikipedia goes on to say that in “Helvetica at 12 point, the em square” is “12 points or 1⁄6 in (0.17 in/4.3 mm)”. So the em-squares of printed fonts should be roughly as follows:
Point Size | Em-square
-----------------------------
12 | 0.17" or 4.3mm
14 | 0.19" or 4.9mm
16 | 0.22" or 5.6mm
But there’s more to the story. Here’s a typographer to share more:
Anyhow, if someone would like to take out a ruler (One with metric markings might be easier to use) and try to measure stuff printed using 12, 14, and 16pt fonts, that might be a good place to start. Just make sure that everything is being printed at 100% and isn’t being scaled or otherwise modified anywhere along the way.
If is in the spreadsheet, there is a scale factor in the page style options.
On other hand, many printer drivers allow to set up an scale factor, verify if it is not different from 100%, or set up up as automatic for make fit the page in the paper if it is not the same size.