Your English is very good!
You can set the default font size from the Learning object title panel (see
techdis-ac.mangoapps.com/sf/MzEzMTJfMTAxNzI2OA
for screenshot).
On the screenshot you can set default text size (1) or if you wanted to add a cascading style sheet to give you more control you could click on Optional properties (2) and select Stylesheet (3) from the bottom of the list then add it to the learnin gobject (4).
But remember that the user can select their prefered font type from the browser and that is generally a better solution because people can then have the font that suits them personally. You may need to help people know how to do that. The following website has good guides -
www.bbc.co.uk/accessibility/guides/allguides_index.shtml