Ok, I answer myself - it was to easy :(
I use it in a Visual Studio Plugin so i had to use PreviewMouseWheel instead OnMouseWheel
private void SQLEdit_OnMouseWheel(object sender, MouseWheelEventArgs e)
{
bool handle = (Keyboard.Modifiers & ModifierKeys.Control) > 0;
if (!handle) return;
if (e.Delta > 0 && SQLEdit.FontSize < 40.0) SQLEdit.FontSize++;
if (e.Delta < 0 && SQLEdit.FontSize > 5.0) SQLEdit.FontSize--;
}