"史无前例的规模" can be translated into English as "an unprecedented scale" or "a scale of unprecedented proportions".
"An unprecedented scale" is concise and directly conveys the idea of something happening on a scale that has never been seen before.
"A scale of unprecedented proportions" adds a bit more emphasis, highlighting the magnitude or significance of the scale.
Both expressions are commonly used in English to describe situations or events that are larger, more extensive, or more significant than anything that has occurred in the past.