Could you please explain to me the following sentence?

"The stranger was six foot two if he was an inch." ???
The overall meaning is "The stranger was at least six foot two, in my opinion".
I agree. The speaker is expressing a strong opinion that the stranger is at least six foot two inches tall.
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Another version of this relates to ages (where you want to emphasise someone's 'extreme' age)

She's 60 years if she's a day!
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It's a way of emphasizing that you are not exaggerating your observation- you make a comparison to something that would be obvious to everyone so everyone knows your observation was accurate-

"He was six foot two if he was an inch" could also be stated "He was six foot two, I'm not kidding"

The expression can be used when describing extreme conditions where the person listening may think you are exaggerating to enhance the story. (I believe) the two extremes in the expression are also used to provide a humerous note to the observation as well.