Hand span is exactly how we would say it. However, if his hands can span the entire lenght of a keyboard (just over 7 octaves on a standard piano) he should be in the guiness book of records for the biggest hands. He has very small hands, he can barely span an octave.
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AnonymousHis hands are very short. They span just long enough for him to be able to reach all the keys of the piano.Following along from the previous post, I think you may mean "His arms are very short". There's nothing wrong with the word "span", but you haven't used it very idiomatically.