Is there a size limit to a CZAR file
There's not an arbitrarily defined size limit that I'm aware of; However, as your database grows, tables that get large enough to exceed a database query timeout (Which I believe may be 60s) during export will be omitted from the exported file.
You may notice this happening if, all of a sudden, your CZAR file shrinks drastically in size; this is an indication that it's no longer exporting a table. You could confirm this by exporting a log file with the CZAR and reading through it.
The first two tables most customers see doing this will be the TrebuchetMail table, which holds a (redundant) copy of every email sent/received in the system forever, and usually Journals afterwards, in cases where customers have marked the Journal. Details field as rich-text (This takes up a lot more space). You would then see other business objects have this issue if they got very, very large and timed out during the export.
However, the benefit to not having a hard limit on size or number of records means that if you are having this issue, you may be able to correct it by improving the performance of your SQL environment in a way that reduces that query time to a level below the timeout (such as adding indexes to tables taking too long to export, or adding resources to the server).