I have a visual basic question about increasing and decreasing font size with two ons.?

So i have two ons and i want one on (btnBig) to increase (by increments like font 12,14,16) up to maximum point like font 16 or decrease (by increments like font 10,8,6 (btnSmall) up to a minimun point like font size 6 of the text i have in a richtxtbox (rtbArticle) does any body know how i can do this?