Click the FX icon, left-bottom of one of the text overlay clips and click on the SCALE effect. Scale the text to the desired size.
Click the fourth button from the left on the toolbar whose pop up reads "Save as Effect Chain Template" and give it a name.
On the timeline, select (highlight) all the text clips to be changed.
On the toolbar click EFFECTS|VIDEO EFFECTS, then scroll down to the Templates section and click the template you just saved.
A prompt should appear to apply to ALL or to ONE. Choose ALL and the template will be applied to every selected text clip.