I open the cell format dialog, choose font, and press “Alt+S” to change the size (because the ‘S’ in “Size” is underlined).
Keyboard focus did not move to “Size” field.
Screenshot from 2013-07-04 18:48:43.png
LO version 4.0.2.2
It must be a forgotten underline. In LO 4.1 (under Windows) it doesn’t exist. Use the Tab key (3 times) to jump to size and then Arrow Up or Arrow Down to select the size.