Hi @Hildweig
For BART max input length  is 1024 tokens. You can think of a token as as a word for simplicity (words can be split in multiple tokens as well). It’s not sentences.
Here document means a seq with max 1024 tokens. Processing longer sequences than that is still a topic of ongoing research.
And I would recommend to read the original Transformers paper (Attention is all you need ) to get an idea about how a sequence is processed by the encoder.  Or the illustrated transformers 3