As it is, I think the question is a little too open, but I can see where you are getting at, and it's an interesting question. Perhaps you could edit to improve what you mean with "limits".

Document size?: None to my knowledge. Total size of all documents?: If documents are stored in the database, you will likely first run into a limit based on the size of the database backup file - otherwise the limit is the size of your filesystem storage capacity. Number of partners? Number of products? Number of invoices? deliveries? purchases? users?: Looking at the source code, there is a hard limit of 2,147,483,647 partners, products, invoices, deliveries, purchases and users.

The web client pages the display of the instances of any one model in groups of 80 by default, but slows down when you change this to display thousands of records.

I am sure there are practical limits that you will run into before the hard limits - based on your ability to maintain data updates from external systems; generating reports (a large number of accounts would take longer to generate a balance sheet than a small number of accounts).