人不可貌相 Not To Be Judged By Looks
國王道：「高徒在那裡？」三藏用手指道：「那玉階旁立者便是。」國王見了，大驚道：「聖僧如此丰姿，高徒怎麼這等像貌？」孫大聖聽見了，厲聲高叫道：「陛下，『人不可貌相，海水不可斗量』。若愛丰姿者，如何捉得妖賊也？」国王道：「高徒在那里？」三藏用手指道：「那玉阶旁立者便是。」国王见了，大惊道：「圣僧如此丰姿，高徒怎么这等像貌？」孙大圣听见了，厉声高叫道：「陛下，『人不可貌相，海水不可斗量』。若爱丰姿者，如何捉得妖贼也？」
逐句解 Line by line
The king said, "Where is your worthy disciple?"
那裡 is the question-word here, nǎ — the fifth place in the chapter where 那 asks rather than points.
Tripitaka pointed with his hand. "That one standing beside the jade steps is he."
And 那 is the pointing one in this sentence, four characters after the question-word in the one before. Nothing on the page tells them apart.
The king looked, and was greatly startled. "The holy monk is of such fine bearing — how comes your worthy disciple to have a face like that?"
丰姿 is fine bearing and 像貌 is what a person looks like. The king sets one against the other and gets exactly the retort he deserves. This is the second time in the space of a page that the court has been frightened by Pilgrim's face.
Sun the Great Sage heard him, and called out in a harsh voice, "Your Majesty — a man is not to be measured by his looks, nor sea water by the dǒu."
Two proverbs in one breath, and the second is the memorable one: 海水不可斗量, you cannot measure the sea with a dǒu. A dǒu is the grain measure, about ten litres of millet — a wooden bucket of a thing, and the joke is the sheer mismatch of scale. The pair is stock and stands in the 增廣昔時賢文 primer and in the 《金瓶梅》, both with 凡 at the head, which the novel drops. 貌相 takes 相 as xiàng, appearance; 斗量 is dǒu liáng.
"If it were fine bearing you wanted, how would the fiend-thieves have been caught?"
如何 sets up a how-could shape, so 捉得 takes the light linking de — against 得寶 two segments on, which is a genuine attainment and takes dé. The argument is the one Pilgrim already made about keeping the fiends alive: judge a thing by what it is for.
Pilgrim has stood through the whole audience without speaking and without bowing, and the first thing out of his mouth is a proverb, shouted. He is not answering an insult so much as correcting a method: the king has just been shown that his three-year inquiry failed for want of looking properly, and is now judging the man who solved it by his face.
The proverb pairs a man with the sea, and both halves say the same thing — the instrument is wrong for the object. You cannot take the measure of a person by looking, any more than you can bail out an ocean with a grain measure.
It is also, from Pilgrim, a piece of self-description he has had a very long time to prepare. The court whispered that he had the face of the Thunder Duke a few sentences ago and did not dare look at him twice. He now tells them, at volume, that they are using the wrong faculty.