Balrogs were almost certainly not actually as big as the one in the movie. I think it probably could have pursued them as long as it wished.
In the movie, it did not make sense for Gandalf to stop on the bridge and challenge the Balrog, because it could almost certainly not have gone out the exit they escaped through, which was just big enough for someone of human or orc size to pass.
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...where the instrument of intelligence is added to brute power and evil will, mankind is powerless in its own defence.
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