You might want to keep in mind that some people use smartphones and it would surely annoy them if half the screen in filled with such a button due to a too large font size.

Elgg's title section buttons (which are usually used for offering and "Add" option) use the "elgg-button elgg-button action" classes to format the button layout. These classes are defined in views/default/css/elements/buttons.php. Increasing the font size can be done by increasing the value of the font-size parameter within the elgg-button class. But then ALL buttons of Elgg will show their text in a larger font! You could limit the larger font size to elgg-button-action type buttons by defining the font-size within the elgg-button-action class additionally. But this will still change the font size within all elgg-button-action type buttons. An even more fine grained definition of font-sizes would mean a much higher effort. You would have to modify the code everywhere where such a button is added within the code specifially.

If you want to change the CSS of Elgg core (i.e. the button classes) nevertheless, you might want to read http://learn.elgg.org/en/2.0/guides/themes.html to learn how to modify the files without changing Elgg core files directly.

Stick to the 1.12 docs unless you're bravely trying out 2.0 :) http://learn.elgg.org/en/1.12/guides/themes.html

I don't know why they're defaulting to 2.0 and I can't change it at the moment.