In Flexitive, there are 2 ways to re-size a text box. You can either re-size the text box from its corner, or from its edge. The copy in your text box will change differently depending on which of these options you use.
When re-sizing a text box from it's corner, the font size will remain the same, but you'll be able to easily change the line breaks for your copy.
When re-sizing from the edge of your text box, the line breaks of your copy will remain the same, but the font size will change.
By correctly using both of these options, adjusting your text to fit your design can be done very quickly.