With VBA in Worksheet_Change.
More detail would be needed to write the code. I don't 'hang about' here regularly. Try a new thread in the Excel Programming section. Describe what it is about the content that requires the change of height to 40. The reverse aspect of the question may also help - what it is about the content that requires a height of 15.

Only a particular column? Multiple columns? The better the detail about the location, the less you should have to do to fit the code to your needs but, don't go overboard (people tend to glaze over on long-winded explanations). Aim for concise.