And you may not have had the right company come out to look. Most good quality companies want to educate their clients, and some will even let you do the ductwork yourself. But only after they size up and do load calculations for what you are planning.
From here there really is no good advice on sizing until you have someone farmiliar with your house look at it.
Chances are it could be fine, but I would still ask a professional opinion. Let us know what they say.