What did i do wrong about font size by using CSS?

Hello there,
could someone give me an advice about font size?
because I have been tried a lot to put number or percentage in font size and it is still said that i did not style the font size yet.

table, th, td { border : 1px solid black; font size: 100%; text-align: center; }

Thank you,

css properties are always a single word, so if the property consist of 2 words (like font-size) the property contains a hyphen.