Highlight the text whose font size you want to increase/decrease (perhaps using some of the text highlighting tips I talked about previously!) and press the key combination
CTRL + SHIFT + > to increase font size by 1 point
CTRL + SHIFT + < to decrease font size by 1 point
(the < and > symbols MAY be on different keys depending on your keyboard layout - they are the two keys to the right of the M key)
On some versions of Word you can also use CTRL+ [ and ], which is even simpler!
This is such a simpler way of changing the font size, especially by just a few points, you'll wonder why you ever used the mouse. And the other cool thing is that you can use this to increase the font size by the same increment across your whole document - even if you have different font sizes in there they will be increased by the same amount, retaining relative sizes.