How is microdontia usually treated?

Depends: Microdontia if it's a undersize tooth you can build it up with composite, porcelain, a crown. If its a extra tooth which is undersized usually it is removed.

Crowns, veneers??: Microdontia is the term we give to teeth that are just real small. It depends on where they are, how many there are and what is the coronal (part you see) and root structure of the tooth. If the number is limited to a few teeth and the roots are good enough and there is enuf tooth to work with..Then you can crown them or veneer them. Only xrays and an evaluation will tell you what can be done.