I am not an expert in phonetics. To me, the "i" in "medium" is "long" in the sense that it has the nature of the vowel in "see" rather than the vowel in "sit". However, it is not "long" in the sense that it is extended for a long period of time.
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I am not an expert in phonetics. To me, the "i" in "medium" is "long" in the sense that it has the nature of the vowel in "see" rather than the vowel in "sit". However, it is not "long" in the sense that it is extended for a long period of time. In actual duration it is rather short.
Someone else may be able to give you a more technical answer.