Assuming that you are using the latest versions, 7.24 or 7.25, there will be an Aspect Ratio drop-down menu next to the split button, under the preview window.
Click the chevron and choose MATCH MONITOR. The preview will then display properly for your monitor. 
Type your text so it doesn't exceed the boundaries of the image area and see if this corrects the problem.
You can also try CROP TO ASPECT RATIO in the effects menu for the image 
or CROP AND ZOOM in the export box when outputting the project, to see if the result is pleasing.
If these methods do not succeed, please share the project so others can take a look. Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/