Some sites for measuring and what aspects to think of
h ttps://www.backcountry.com/explore/how-to-fit-a-backpack (short video on linked site)
My personal advice would be:
What role has this backpack, be carried extensively or just from car to work, for casual stroll with light load?
What shall it keep in volume and weight? What is the largest Item (a 17# laptop for example)? What is the heaviest Item?
If you have this cleared go/drive with your wife to a big & good brick and mortar outdoor store, let her choose what she likes from quality manufacturers, have it properly adjusted by the personal, loaded up with aprox. the maximal intended load and carry it at least 15 better 30 minutes. It may suck mildly to be carried, but should not hurt.