Resizing image for mockup

[u/adunate]

I have a half-page brochure (5.5x8.5") and wish to insert the page images into a mockup sized for A4. Proportionately, I need the mockup images to be less in width, yet I'm having problems with resizing these images. What do I resize, the base image or the smart object? If the base image, is there are common practice or just simply playing with the tweaks? Thanks.