Chibiusa goes through several different designs in the manga as she grows up, but she's almost always wearing some school uniform or another. She's also unusually short for her age, a point that you notice when she hangs around with anyone from her class. When Chibiusa first shows up she's in fourth grade at Juuban Elementary. During the Death Buster's arc she's elected class president of the fifth grade, and at the beginning of the Galaxia arc she and Hotaru are both in sixth grade (classes 1 and 3, respectively).
This is Chibiusa's dark queen form, brought about by Wise Man. In this form he calls her his dark queen and she sits beside him on the throne of Nemesis, although she is occupied entirely by 'Endymion,' who is in actuality the brainwashed Tuxedo Kamen. Because of her affinity with the ginzuishou she has immense evil powers. Eventually it is the sacrifice of Sailor Pluto and her death which drive Chibiusa to wake up into her true self, Sailor Chibimoon. She cries one single tear which flares into the ginzuishou, and the curse is broken and she transforms for the first time. As Black Lady Chibiusa stands just slightly shorter than Sailor Moon, her own taller odango coming up to crest at the same height as Sailor Moon's shorter, rounder ones. Otherwise she seems to be mature adult in body, although she remains a twisted, immature child on the inside.
This is Chibiusa's pretty sailor suited warrior-in-training form that she finally attains near the end of the Crystal Tokyo story. Her primary color is a deep pink and her secondary is red. She wields the Pink Sugar Heart Rod and is master of the devastating Pink Sugar Heart Attack as well as the one time used Moon Princess Halation. She transforms with the power of the Mirai no ginzuishou in her Prism Heart Compact.
Super Sailor Chibimoon
This is the form that Chibiusa first attains by summoning the future Holy Grail. Later when she can no longer summon the future Grail, Elios helps her power up and she attains the form again. As Super Sailor Chibimoon she first wields the Pink Sugar Heart Rod then later the sentient Chibi Moon Kaleidoscope. Her signature attack as Super Sailor Chibimoon is Moon Gorgeous Meditation, usually in tandem with Usagi.
Princess Small Lady Serenity
Chibiusa's unique princess form wears a long multiple layered dress of pink gauze. While in this form she also manifests the crescent moon on her forehead. Once Chibiusa attains her first henshin and unlocks her power, this is the form she always naturally takes in the future and takes in the past during moments of importance (like the coronation of the King and Neo Queen).
Once again, Chibiusa only holds this form for a little more than a day. She gets the body of a fifteen-year-old girl due to a curse that Palla Palla levels at her. At first she's very impressed with her grown-up body and acts as grown-up as she can manage, but later she realizes that she wants to grow up herself and not get there through magic. Chibiusa and Usagi's joint henshin breaks the curse over them and they return to normal.
Sailor Star Chibimoon
Chibimoon's final (presumably) sailor soldier form. When she attains Star form at the same time as Usagi attains Eternal she finally comes into her own as a sailor soldier fully. Her ginzuishou blooms into the Pink Moon Crystal, and her court, the Amazon Senshi, appear to her. In this form she uses Pink Lady's Freezing Kiss, and it is also the form she holds while fighting Galaxia with Eternal Sailor Moon.
Princess Lady Serenity
This is the vision that visits the dream priest Elios before the invasion of the Dead Moon. She appears very indistinctly and tells him that he must search for one who is a princess and a soldier, guarded by the moonlight, who dreams a beautiful dream. Only she can save the Earth and Elysian from the impending threat. She identifies herself only as "Princess Lady Serenity," but even in the haziest and most indistinct images, she's pretty clearly Chibiusa, properly aged to about the same state as Black Lady, only naturally, and in due time. Presumably this is how she looks even further into the future, past her appearance in the last three volumes as Sailor Star Chibimoon. She appears to be wearing the same style of dress as Princess Small Lady Serenity.