It is not font size that is affected. If you do not have the font installed that was used for the presentation, then it is a matter of setting a substitute font that has similar metrics as the original font. Indeed, depending on the metrics of the font, one font may take substantially more place than another font, and even look larger, at the same font size.
Under Options, LibreOffice - Fonts, you can define which fonts should be substituted for a font that you do not have. To activate the Replacement Table, check "Apply replacement table". From then on, you can define fonts, indicate replacement fonts and add these to the table.
Each font is unique, and therefore, the most suitable substitute font depends on each individual font. There is "general" way to automatically set the best substitute font (which, moreover, depends on the fonts you have on your system.
Substituting two common Microsoft fonts
If you have to work with documents coming from a Microsoft Office product, changes are high that it uses either the Calibri or Cambria fonts. These are the regular sans-serif and serif fonts of MS Office, introduced in 2007. In contrast to the previous "generation" of Microsoft fonts, that included "Arial" and "Times New Roman", the license of these newer fonts is more restricted. Therefore, these fonts cannot be used freely on Linux. Google produced two fonts, Carlito and Caladea, that have similar metrics of the two commonly used Microsoft fonts, and therefore can be used as substitutes for these. They can be installed on Ubuntu with the command sudo apt install fonts-crosextra-carlito fonts-crosextra-caladea. (Thanks to user alci to point this out)
