There is no hard limit on data base size. There is a limit on list sizes (items, customers, vendors, etc). They are limited to around 10,000 entries - its a little higher but I don't recall the exact number off the top of my head.
As the database grows very large performance can decrease. The impact depends a lot on the data in the company file. In my experience it is most impacted by large number of inventory transactions perhaps because QB has to calculate the Avg Cost of Goods Sold repeatedly and many items slows that down.
This hasn't been quite the issue it used to be as computers have gotten faster and in particular as SSDs have come to replace physical hard drives. We have also done quite a bit of work at speeding up QuickBooks Mac so newer versions are more performant.
But if you aren't hitting slowdowns then you can keep adding data without limit. All supported versions of the program are already 64 bit which gives it a very large address space.
