Hi Katarina,
this page might help config.json · Helsinki-NLP/opus-mt-en-sla at main
I think this says that the maximum number of tokens, max_length, for this model is 512.
512 tokens might correspond to about 2500 characters (~letters), which might correspond to about 400 words.   This is a very rough approximation, and different texts will have different conversion values.
If you want to know more about tokens, there’s a nice introduction to BERT tokens by Chris McCormick BERT Word Embeddings Tutorial · Chris McCormick (I imagine that the Marian model uses something similar).