Question: In one of my cases, the interviewer used industry specific metrics like sales per square foot in a retail case. What are some common metrics that I would need to be familiar with?
I wouldn't worry about this too much. There are too many industries and too many metrics to spend time familiarizing yourself with all of them. They are all however, just variations of the same thing.
In health care, they aren't customers they are patients. In life insurance they aren't customers, they're called "lives" and in we have 100,000 lives insured (e.g., we have 100,000 policy holders).
Revenue per customer, revenue per patient, revenue per life (insured) is all the same thing... revenue per customer. No need to memorize the specific variation, just ask for the generic version and the interviewer will generally give you the more specific version.
In looking at revenue productivity, you can look at revenue per sales person, revenue per reseller, revenue per square foot, revenue per store. This is all just variations on the same core theme. Bigger distribution assets produce more revenues, so to compare sales productivity on an apples-to-apples basis, you need to look at revenues per SOMETHING. The "something" varies but the circumstances of the case but the need to look at revenues on a "revenue per SOMETHING" basis often does not.
I never bothered trying to memorize these specific terms, never held it against a candidate if they used the generic phrasing of a metric instead of the more specific (since that happens all the time on the job where you need to learn the industry terminology and reporting conventions), and never felt like an interviewer held it against me for not knowing the industry specific phrasing of key metrics.
In short, my advice is to spend your limited time practice something else other than this.