I'm not sure why eliya.joy says it's not (technically) correct to use גדול as 'great', as it has been used in such a way from ancient times. For instance, הכהן הגדול was the High priest of Israel, the Great Wall is החומה הגדולה and כנסת הגדולה - the Great Assembly.
But if you want a different word, for people you can also use the adjective דגול, which means famous, important and admired.
He probably thinks it sounds slangy because of its usage in things like !תותח, אתה גדול, אין עליך, or in that song people sing, הוא גדול הוא גדול הוא גדול: sorry about your ears, anyone who listens to the clip, I can't believe I could only find this one! :D
That's not slang. From wikipedia: "Slang is language (words, phrases, and usages) of an informal register that members of special groups like teenagers, musicians, or criminals favor (over a standard language) in order to establish group identity, exclude outsiders, or both."
Sorry, had to delete my response because the Hebrew letters mixed in with English were wreaking havoc on word order.
Hebrew has two silent letters: א ע
All Hebrew letters are consonants, although there are two that can also function as vowels: י ו
Apart from those two, vowels are rarely written, and appear as pointing above and below (usually below) the consonants. So any vowel sounds you hear are associated with that not-always-shown pointing, rather than the letters themselves.
They aren't silent, they are glottal stops, which are consonants. Transliterate these English names to Hebrew: Ann, Evan, Ian, Oliver, and we see that they all start with א. Also English rarely has glottal stops in the middle of the word ("Hawai'i" is an example), but Hebrew is full of them.
The א in אישה is not an "A sound", but rather the glottal stop in front of the vowel. אישה is "eesha" while ישה would sound like "yesha".