Turn on copyfitting

1. Click the text box.
2. On the Format menu, point to AutoFit Text.
3. Do one of the following:
   • To shrink or expand text to fit in the text box when you resize the box, click Best Fit.
   • To reduce the point size of text until there is no text in the overflow area, click Shrink Text On Overflow.

After this, whenever you resize the text box, the font size remains the same.