Not sure about the size but what's wrong with the stock cooler? If the cooling system (both engine & tranny) is maintained, it's more than enough to cool a properly operating Montero.

Off Roader
98 Montero with the Winter Package
89 Montero minty clean and reserved for overlanding trips or Cars and Coffee events
96SR (3.15:1 xcase, 35's) gone to the rust gods
96SR Build Up Thread
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Old web page