Can a dentist increase the size of my tooth or give me a fake one?

Yes: I'm assuming you have 1 or 2 teeth that look smaller than the others. A dentist can make the tooth look bigger. You would need to see a dentist to evaluate and give you all the options. Some possible options are: crowns, veneers, and bonding.

Definitely: Whether it's replacing a missing tooth with a "fake" one, or making a natural tooth look larger, it's all doable by your dentist. It could be done with a tooth colored resin or bonding, or a porcelain veneer or crown. Just talk to your dentist and let him/her know what you're looking for in terms of the final result.