In the old days, Parse had a limit of 128kb as a max size of a PFObject. Is this still the case (for a server with a mongodb/atlas db?)? If it is not still the case, is there a max size and if so, what is it?
That restriction doesn’t exist anymore. It all depends on your Database Limit and RAM.
Here is Mongo Limit
Like @dplewis answered, there is no specific limit anymore, but I’d suggest you to avoid very big objects because it can turn out as a performance issue later. I am not sure what you are thinking to store in the object, but depending of the kind and size of data, I’d prefer to use files instead of a byte array or a long string field or even a very long object that you don’t need to search within. Even if you decide to go with big objects, make sure to use select() method on your queries to only return the data you really need in each of the queries.