I want to play an aasimar and was wondering why they always look like a human. An aasimar or a tiefling can also be a halfing or a gnome or whatever. My question is, are there any rules how to create an e.g. Aasimar Halfling? And if not, how do you solve this?
The standard Aasimar is a human-celestial hybrid, but there is a sidebar on the SRD that describes non-human Aasimar. In short:
Non-human aasimars have the same statistics as human aasimars with the exception of size. Thus a halfling aasimar is Small but otherwise possesses the same statistics and abilities as a human aasimar—the difference is purely cosmetic.
Effectively, you use all of the racial rules for an Aasimar and just change your size to match your mortal half. This means that you get a +1 size bonus to your AC, a +1 size bonus on attack rolls, a –1 penalty on combat maneuver checks and to your CMD, and a +4 size bonus on Stealth checks. Your speed doesn't change.
One important thing to note:
It should be noted that while any creature that breeds with a celestial may give birth to half-celestial offspring, only humanoids can give birth to aasimars.
So, while you can have Small Halfling Aasimars or Gargantuan Rune Giant Aasimars, you can't have Dog Aasimars. A partially-celestial non-humanoid born on the Material Plane is likely a Half-Celestial. I would guess that the designer who wrote that any humanoid race can be an Aasimar with a different size wasn't aware that giants are humanoid in Pathfinder.