The IRS looks at the "reasonable salary" thing in comparison to distributions (dividends) an owner of an S-Corp (or LLC) takes.
Salary is subject to Social Security and Medicare withholding ("payroll taxes"), and with company matching it comes to 15.3%.
Distributions/dividends are not subject to these payroll taxes...only federal and state income taxes.
So, the IRS doesn't want employee owners of an S-Corp/LLC taking a disproportionate amount of distributions versus salary, as they can actually consider it SS/Med tax evasion!
A good accountant can clue you in on what is an acceptable ratio of salary to distributions, but common sense goes a long way here. For example, taking $50k in distributions and only $25k in salary for several years in a row could get you on their radar...I would think?!
So, in summary, a "reasonable" salary can be anything you decide, the government can't tell you how much to pay yourself.
But, if you take more than a "reasonable" amount (say 1/3??) of your pay in distributions, (especially in multiple years), the IRS can go back and declare the "excessive" distributions as normal salary, and bill you for the SS/Med, fines, and late fees.
What ultimately constitutes "excessive distributions" by the IRS is up to them...I reckon...??