My two possibly tallest OCs are either Montie or Awenasa, but there are strange caveats with them.
Since Awen has the lower half of a snake, if she wanted then she could bring her height up to 14 feet, but to keep things fair she is technically only 6 feet from the hip up. But, if you wanted to include the base of the tail - in a basic standing position - then that brings her height to around 8 feet.
Montie is quadripedal, so she has to be measured by height at the shoulder. In that case she's also 6', but if you measure to the top of the head when the neck is at a 90° angle, she would be around 9 feet tall. And of she extended her wings and pointed the tips straight up, it would probably reach about 14 feet.
So... I honestly have no idea.