The above speakers have already explained the meaning, now I'll try to explain why the verb “生气” is separated. In Chinese, there are a large number of verbs that can be separated, they are referred to as "离合词" (li2 he2 ci2). The verb is separated for reason of modifying it. “生很大的气” the modifier is put inside the verb. I'll write a couple of other examples to get you familiarised with the concept: 我们明天见面 我们昨天见了面 我们见过三次面 我生了他的气 你生什么气啊？ 我生了她很多次的气 吃什么反？ 他已经结了婚 As with most concepts in the Chinese language, you'll fully understand then first when you've been exposed to them a number of times. But I know, this one is a bit tricky! Hope it helped.

生气 means become angry, as you know. Separately, 生 as a verb, means generate/become, and 气 means anger. literally, 那么大 means "that big", "so big", stressing on the severity of anger. In addition, 何必 implies that you don't need to , or don't have to do sth, and is always followed by something bad for the person addressed. It's used in rhetorical question. Literally, the sentence means "why are you generating so much anger ?" or, why are you so angry? But, because of 何必， it's not really an open question that asks the reason, but actually tries to persuade you that please don't be so angry.

生 is a verb, 氣 means anger

it means"you should not so angry" .if you are very angry for my joke,but i think i was just kidding.i would say" 你何必生那么大气？（why are you so angry?）"。in this sentence,"那么大"=very

It just means "why are you SO angry"? Or literally, why are you generating such a large (那麼大的) QI?