What happened to using your mare for Grand Prix? That is a huge about-face in less than 48 hours!!
Spyder is right, in order to do any of the aforementioned goals you really need to focus on one main goal. On the other hand, there is no shame in doing dressage to the best of your ability without making it to Grand Prix, there are precious few that make it that far.
As for size of a foal, it is hard to estimate the chances of it being big. There are no guarantees with size, especially with a first foal.
All horses deserve, at least once in their lives, to be loved by a little girl.