Create a rectangle with stroke and no fill in front of the photo with the desired dimensions for a reference or as a guide and fit the width of the photo in it in by scaling it proportionally. You have to decide which parts along the image's height will be masked out. After that select the rectangle and photo and apply Clipping Mask.
The other ways of fitting is to stretch non-proportionally the image, or repeat it horizontally which is unlikely to be acceptable in most cases.
edit: Hi Larry,
@ross, as Larry suggested instead of the rectangle and masking, use the Artboard and the parts of the image outside of it will be clipped.
