で言うと

The phrase で言うと can be used to relate something to a different system or framework, as Tobira says. Examples include a different unit of measurement (in to cm), different languages, or different types of calendars. It often reads as "in terms of", "according to", or "from the viewpoint of."

今の気温は華氏で50度だから、摂氏で言うと10度ぐらいだ。
The current temperature in Fahrenheit is 50 degrees, so in Celsius it's about 10 degrees.

西暦2000年は平成で言うと平成12年に当たります。
The year 2000 AD in terms of Heisei would equate to 12 Heisei.

僕の身長は6フィート、メートル法で言うと約180センチだ。
I'm 6 feet tall, and according to the metric system that would be 180 centimeters.

病気の人に言う「お大事に」という表現は、英語で言うと "Please take care." に当たります。
The expression 「お大事に」that you say to sick people is equivalent to "Please take care" in terms of English.