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Full Idea
'Jack could have been taller' implies a different Jack in a different world, so Lewis defines a counterpart in a possible world as an individual sufficiently similar to Jack, and more similar to Jack than anything else in that world.
Gist of Idea
A counterpart in a possible world is sufficiently similar, and more similar than anything else
Source
report of David Lewis (On the Plurality of Worlds [1986]) by Thomas Mautner - Penguin Dictionary of Philosophy p.115
Book Ref
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.115
A Reaction
If we say something like "I could have been twins" or "I could have been a genius" in another world, it would need an odd concept of my personal identity for it to remain identical in those counterfactual situations. Lewis has a point.