I can't answer your question fully, but I understand Access has a 2GB size limit. As to slowing down - yes, the more data you add the slower it will get, but, it will work significantly faster than a non-true-database product such as Excel with a similar number of records.
How fast things run depend entirely on how much those 5000 records add to the size of the file and what exactly you are running. I'd guess that you'll not notice much of a slowdown until you get above at least 50mb in size.
Mike