諕矮了三寸 Shrunk At The Sight
這獃子就諕矮了三寸，道：「爺爺呀！眼有這般大啊，不知口有多少大哩。」行者道：「賢弟莫怕。你兩個護持著師父，待老孫上去討他個口氣，看他是甚妖精。」八戒道：「哥哥，不要供出我們來。」这呆子就諕矮了三寸，道：「爷爷呀！眼有这般大啊，不知口有多少大哩。」行者道：「贤弟莫怕。你两个护持着师父，待老孙上去讨他个口气，看他是甚妖精。」八戒道：「哥哥，不要供出我们来。」
逐句解 Line by line
At that the idiot shrank three cùn shorter with fright. "Grandfather!
A cùn is the Chinese inch, about an inch and a quarter. 諕 is xià again — the third and last of the chapter's three appearances, and automatic transcription is wrong at every one. 爺爺 takes an unstressed second syllable, like the whole class of doubled address terms.
"If the eyes are that big, there is no telling how big the mouth must be."
The inference runs from a part to the whole — exactly the method the elders used when they measured Pilgrim against the creature's teeth.
Pilgrim said, "Do not be afraid, worthy brother.
Seven characters, but the unit carries a full statement after the bracket and so is not a bare opener. It stands as its own segment.
"You two guard the master while old Sun goes up and draws a word out of him — we shall see what sort of demon he is."
討口氣 is to sound somebody out, to get a word out of them — the same thing Pilgrim has been doing to the villagers all evening. 甚 is the question-word shén, and 著 the marker zhe.
Eight Rules said, "Brother — don't give us away."
供 is gòng, to inform on somebody or confess — a courtroom word — not the gōng that means to supply. 哥哥 takes the unstressed second syllable.
Three cùn is a measurable quantity of fright and the text reports it as fact. It is the same trick as the elders' remark about the teeth: something that cannot be quantified, quantified anyway.
Pilgrim's plan is reconnaissance rather than combat. He will go up and get the creature to talk — which is what he has been doing to the villagers all evening, asking questions until he has the whole account.
Eight Rules gets the last word and it is the funniest line in the exchange. He does not ask Pilgrim to be careful, or to come back. He asks him not to mention that the rest of them are down there.