I work at Facebook and I can give a definitive answer about this.
Please don't put a maximum size on the storage for an access token. We expect that they will both grow and shrink over time as we add and remove data and change how they are encoded.
We did give guidance in one place about it being 255 characters. I've updated the blog post that had that information and updated our new access token docs to include a note about sizes:
Sorry for the confusion.