三百餘里 Three Hundred Li Straight Down
大聖伏在洞邊上，仔細往下看處，咦！深啊，周圍足有三百餘里。回頭道：「兄弟，果然深得緊。」八戒道：「你便回去罷，師父救不得耶。」大圣伏在洞边上，仔细往下看处，咦！深啊，周围足有三百余里。回头道：「兄弟，果然深得紧。」八戒道：「你便回去罢，师父救不得耶。」
逐句解 Line by line
The Great Sage lay down on the lip of the hole and looked carefully down it — and oh!
Careful attention, and then a sound. 仔細 is read zǐ xì, and in it 仔 is zǐ rather than the zǎi of a young animal. The one-character exclamation at the end is the storyteller dropping out of narration entirely for a moment.
Deep! A good three hundred lǐ and more all round.
Three hundred lǐ is around 173 km, so this is not a pit but a country. The measure is 周圍, the way round rather than the way down, which quietly tells you the shaft opens into something enormous.
He turned his head and said, "Brothers, it is deep in earnest."
Here 得 is de, the light syllable hooking the adjective to its measure, and 緊 means tight and by extension very much so. Pilgrim has just conceded the argument.
Bajie said, "Then you had better go home. The master cannot be saved."
The closing 罷 is ba, the light suggestion particle, and in the second clause 得 is de, the can-or-cannot syllable. Bajie is proposing to abandon the mission for the second time in this chapter, and this time he has a measurement to point at.
The depth is given as a circumference, which is the detail that turns a hole into a world. Three hundred lǐ around is something like 170 km, and this is what Pilgrim can see from the rim.
The Bottomless Cave will turn out to have daylight in it, wind, flowers, fruit trees and a household. The novel is not describing a pit; it is describing a place that happens to be entered from above, and Pilgrim is about to say so himself in almost admiring terms.
Bajie's second desertion attempt is better argued than his first. At the well he wanted to divide the luggage on a rumour; here he has a number. It does not help him.