Everyones experiance is different. Sometimes inproper rideing technique will lead soemone to feel a bike is heavy when if they would take the time to learn good low speed rideing the same bike wont.
My guess is the person you were talking to isnt as good of a rider as they think or is trying to guestamete your ability based on (first BIke questions) many people belive the first bike should be real small no matter what.
So with that being said, I dont see why your size would prohibit you from the thunderbird. I am 6'2" and about 212lbs and find the thunderbird just about perfect for me. However your skill level will have to be coinsidered and this is something I cant do on a computer. Its also something i wouldnt base on the time you have been rideing but rather how well you can ride/handle a bike that I personally see you ride! Then and only then could I make a comparrison based on your size-vs-bike-vs-experiance and state weather or not its a good fit for you.
based on past experiance helping new riders My first instinct would be to say no its not a good first bike but as in the past there is always the exception!
So let us have the info needed to help you better. How long have you been rideing? what have you ridden? have you had any professional training? Do you have good long term riders to help you along the way? If so what bikes have they recomended as your first bike?