What is the smallest gauge needle that fat can be injected through?

Fat should not be injected through a needle because of the risk of injecting it into a vein.  A canula is used since these do not have a sharp tip.  If the canula is very small it can be hard to inject the fat.  Usually a 23G or larger is used.    

Fat grafting should not be injected through a needle. Needles are sharp and have greater potential to damage blood vessels and nerves as well as increased bruising. For facial work I use a tiny 18 gauge tapered end injection cannula. I do use an 18 gauge needle to make a tiny opening, about a millimeter in length, in the skin to allow the cannula to enter. For body injection I use a 

Fat grafting should be performed with a blunt tip cannula. In order to determine that a plastic surgeon can provide beautiful, natural looking and lasting results, look for many before and after images. Fat grafting requires expertise, considerable training, specialized instrumentation and patience on the part of a surgeon. 

Fillers and fat transfer to the face should be with a cannula. Smallest is 25 or 27 gauge.The art is in the injector not the instruments. How fat is harvested and prepared and then the pure fat injected artfully.

I use a sharp 23 gauge needle for injecting fat along the vermillion border of the lip for lip augmentation.  Otherwise, for all other facial locations a blunt needle is preferred, usually an 18 gauge needle.