Specifically, the authors found that the modern era is unique in the extent to which creatures with larger body sizes are being preferentially targeted for extinction.

具体来说，作者们发现，现代是独一无二的，因为体型较大的生物在多大程度上被优先指定为灭绝目标。

the extent to which creatures with larger body size为定语从句修饰前面名词“extents”并在定语从句作程度状语，关系代词which指代先行词“extent”结构为“to some extent在某种程度上”。