Just to expand on Brian’s advice, Capturing the image in Clip mode will produce an image the same size as your video frame size. If you are using HD video I don’t think there is any problem.
If using standard video 16:9 then you may have problems with your image looking a little distorted.
Project Playback mode……
If you were to save the still image in Project Playback mode using Tools – Save Current Frame as Image, this will create an image as you see on the preview screen which is I assume what you want.
So can you give details of your video file.
Right click the clip in the timeline and select properties.