What is a size of character?

The size of character is specified by the Font size selected. It is there in the Home tab (Word 2010 and 2007).

To change the font size :-

STEP 1- Select the text you wish to edit. (holding down mouse left button and dragging across the text)

STEP 2- With text selected, go to Fonts size option under the HOME tab in ribbon- Clicking it will open up a menu list of font sizes.