At export, the auto-selected frame rate, default quality and resolution are likely okay as is. 4:3 is no doubt the source aspect ratio.
To fill the frame, try Crop and Zoom.

Experiment to see what looks best.
If you take a minute to share a few of these pics, someone here may be able to offer more specific suggestions...
Upload the file/s to a free server, such as Google Drive or MS OneDrive, get a shared (public) link, copy the link and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum. When using Google Drive, if necessary change "restricted" to "anyone with link can view."