how many characters of description a user story (EPIC)
CA Agile Central has a 32k character limit for rich text fields (Description, Notes, etc.).
Please let us know if you have any further questions.
To elaborate on Michael's answer, you can find this and all other field definitions in the WSAPI documentation. For example, the Description field is defined at the Artifact level and you can find that at https://rally1.rallydev.com/slm/doc/webservice/objectModel.sp#Artifact. Hope that helps.