Changing the font size for customized text labels

Is it possible to change the font size in a customized text label?  I noticed in the player, you can change the message for player text labels, but I don't know where to look for changing the actual font size.  The title font size is smaller than the message font size.  I would like the title to be larger than the message font size.  See the attachment (when you click the "Submit" button before finishing the activity, a system dialog box says "invalid answer" - this is the dialog box that I want to change the font size).  Thank you for your help!