I would not use a gray card use a white card for balance. This will allow you to see any color cast. If you think about it, the white balance in your camera is just like what video guys have been facing for years. They always use a white card. However make sure it is pure white. Alot of white card can either have blue or yellow in them.
Any size card will work. Shoot on frame with the card and then take it out and balance.
Any size card will work. Shoot on frame with the card and then take it out and balance.