One can also define additional keystrokes in the Writer for increasing and decreasing the font size by going into Tools | Customize, picking the “Keyboard” tab, in the top half (“shortcut keys”) picking a keystroke you want to have a new action, and in the bottom (“functions”) picking the action (e.g., Format | Reduce Font, or Format | Increase Font). (The “customize toolbar” option for Calc doesn’t seem to have an increase font size / decrease font size icons to select.)
You can also change where they appear on the toolbar once they are selected. About 80% the way down the list are places where you can check for the icons for increasing and decreasing the font size. If you right+click anywhere on the Format toolbar, and then select “Customize toolbar…”, in the “Commands” window is a long list of formatting commands with a place to check in front of each one. The icons for increasing & decreasing the font size, A^ and A v are apparently by default not displayed. One can also use the pull-down for the style (and even customize a style) that sets a different font size. On the formatting toolbar, one typically has (among other items) the name of the style to be applied, the current font name, the font size, then icons for Bold, Italic, **U**nderline, … One can use the pull-down for the font size to change the size of the font.
It looks like the default keystrokes for increasing the font size and decreasing the font size are Ctrl+] and Ctrl+[ respectively. I am not that familiar with other applications in LibeOffice to be able to comment about them. This explanation is for the Writer in version 4.0.2 (and, parenthetically, whether they apply to Calc).