It sounds as if you are saying that the dimensions of the video are too small. In our suggestions for conversion, we asked you to use "Keep Source File Dimensions". If you selected this option, the video that you are importing has exactly the same dimensions as your original video.
To increase the frame size of your video, open the Custom MPEG 4 Properties and select "Change Dimensions". Use the Video Dimensions drop-down to select a larger size. Be sure to leave "Lock Aspect Ratio" checked, to avoid changing the appearance of your video.