Look at the various When-Mouse- triggers, for example,
When-Mouse-Click Trigger
When-Mouse-DoubleClick Trigger
When-Mouse-Down Trigger
When-Mouse-Enter Trigger
When-Mouse-Leave Trigger
When-Mouse-Move Trigger
When-Mouse-Up Trigger
then use the set-item-instance-trigger to change the visual_attribute which you will have defined as having a 'higher' field size and 'bigger' font size. Remember to tunr them back after you leave the field.
You may need to do a 'synchronize' to force it to show the new characteristics.
David