"no bigger than the tip of his wife's little finger" That's actually kind of big for a pearl.
A phaeton is actually a big deal. The types and number of carriages that Darcy and Elizabeth would own together would have been outlined as part of their marriage contract.
And Darcy doesn't actually do anything because he's concerned about scandal. Well that's a refreshing line for a fanfic to take.
Author's response
1) Just the very tip, not the entire first joint of her little finger. That would be a decent size for a pearl, but not enormous or gaudy.
2) Yes, the gift of a phaeton and ponies was meant to signify a large and extravagant gift, contrasted with the smaller gifts of a book, a ribbon, etc. Elizabeth teases him about them all, regardless of the cost was what I'd meant to indicate. My apologies if that wasn't clear.