At any point while you're transcribing, you can stop and see what you've done so far. You can click on the Preview button and your video will stop playing and a movie player window showing your captions over your video. This window is meant to give you an accurate idea what the Open-Captioned Movie export will give you. It may not be a good representation of other caption formats such as the SCC format, which has its own style, which is white text over a black background.
Note that the Preview window is geared towards giving you as fast rendition of your captioned video, and so it creates the caption from your list on the fly. If you select a caption in your caption list, then click the Preview button, it will show that caption briefly, then it may not show the next couple of captions or so until the captions catch up with the video playhead. Starting from the beginning is usually best or select a couple captions before the one you want to see. It may also not show the last caption in your list if you are not done transcribing the whole video. This is simply a current limitation of the Preview window and will not affect your final exported movie. All captions will be included in the export.
The Preview window will try to open your movie at its native size. If the movie is too large to be displayed on your screen, it will automatically reduce the movie's size so you can see your captions. This will not change your original movie's size. It's merely a way of allowing you to see the captions on large movies. When you export, your movie will not change size at all.
If you see that your captions are running off the movie, you'll need to add a line break marker, which is by default the "|" pipe character above your Return/Enter key on your keyboard. This will tell MovieCaptioner to make a line break there, but that character will not be seen in preview or the final exported files. This is due to the Return/Enter key being used to record a caption while you're transcribing for maximum efficiency.
If you see that your second or third line of your captions is not showing up or getting cut off on the bottom, close the Preview window and go back to your Text Properties and make the Background Height setting larger. Your background will vary with whatever text size setting you are using and how many lines are in your longest caption. It may take some trial and error to get it right. Be sure to check your longest caption to verify that it will fit in the background.