Hey, welcome to the boards!
There is a stage size code, for certain stages, made by Y.S. if I recall correctly... But I have no idea where they put it.
We can't add hitboxes at the moment, but we can modify them. If you go on the Brawl+ IRC, I'm sure you'd find someone kind enough to get the hitbox data for Captain Falcon's Dair. Maybe you'll be lucky and there's already a hitbox in that location. As it is, I don't think it's something we're considering changing in the Brawlplusery set, though, because Falcon can combo into and out of the sweetspot just fine.
There is a stage size code, for certain stages, made by Y.S. if I recall correctly... But I have no idea where they put it.
We can't add hitboxes at the moment, but we can modify them. If you go on the Brawl+ IRC, I'm sure you'd find someone kind enough to get the hitbox data for Captain Falcon's Dair. Maybe you'll be lucky and there's already a hitbox in that location. As it is, I don't think it's something we're considering changing in the Brawlplusery set, though, because Falcon can combo into and out of the sweetspot just fine.