Any client or relay could censor you, but it's unlikely every single one of them would. Even if that did happen, relays and most apps are open-source so you can run your own relay that keeps your notes, and others can connect to your relay to read them. Likewise you can run the clients on your own computer and make edits to the code if needed.

I don't think there is a limit hard-coded anywhere, but most relays will impose a maximum note size (i.e. 1MB of text) and/or ban someone who is posting a new note every second (thinking that it might be spam).