～50 mm

Retinal ganglion cells (RGCs) are specialized projection neurons that relay an immense amount of visual information from the retina to the brain. RGC signal inputs are collected by dendrites and output is distributed from the cell body via very thin (0.5-1 μm) and long (～50 mm) axons.

Ref: 

Retinal ganglion cells: Energetics, compartmentation, axonal ...

PMID: 23891817, DOI: 10.1016/j.preteyeres.2013.07.001