The difficulties and risks of the air offensive (that) Goering had so lightly agreed to undertake loomed larger than he had given any hint of perceiving (that they would be).

划线部分为句子主干，主语+loomed不及物动词+larger形容词比较级作状语。

蓝色部分为air offensive 的定语从句。

绿色部分为比较状语从句。该从句后面省略了hint的同位语从句that they would be. they指代主句主语the difficulties and risks.

比较分句可以简化为 than he had perceived (that they would be).

比较分句的意义接近于 than he had thought (that they would be).

翻译：戈林不假思索地同意承担的空袭任务的困难和风险，比他所能觉察到的还要大。