The grammar structure で言うと is used to specify a category, framework, or context within which something is being discussed. It translates to "in terms of," "speaking of," "when it comes to," or "if you put it in." It is formed by attaching でゆう言うと to a noun that represents the domain or standard being referenced.
The most common usage is to narrow down or reframe the topic by specifying what category you are talking about.
で言うと is used when converting or rephrasing something into a different language, unit, or equivalent.
It can also be used to specify whose standard or what perspective you are speaking from.
The form でいえば言えば has a very similar meaning and is often interchangeable. でいえば言えば carries a slightly more hypothetical or conditional nuance ("if we were to speak in terms of…"), while でゆう言うと is more straightforward ("speaking in terms of…"). In practice, both are used with little difference.