"史无前例的规模" can be translated into English as "an unprecedented scale" or "a scale without precedent".
"Unprecedented" is a commonly used adjective meaning "never done or known before," which directly conveys the idea of something being "史无前例" (unprecedented in history).
"Scale" refers to the size or extent of something, matching the meaning of "规模" (scale or magnitude).
Both expressions are natural and accurate in English. For example:
"The event was held on an unprecedented scale."
"The project was carried out on a scale without precedent."