Conforming image size for 2 images

I am trying to align 2 images using Photomerge.  One of them appears larger than the other when viewed in Photoshop, but has a smaller pixel width than the other.  Both images are shown below.  I'd like their width to conform if that helps them to merge into a single image.

The first image has a width of 29643 pixels.  The 2nd has a width of 22435 pixels.  As can be seen, the canvas for the 2nd image takes up more of the screen.  Why is its width smaller?  Is the pixel width the size of the canvas or of the image?  How do I make these 2 images have the same pixel width - just assign it in Image Size?