There are two limits. One is fixed limits in various tables and indices. That would for example be 500 phone numbers on some phones and 30 wallpapers on other phones. BitPim always works within these numbers, such as only writing the first 500 phonebook entries or the first 30 images. Most are also part of the phone spec - see that at www.phonescoop.com or your phone manufacturers website. If neither phonescoop nor your manufacturer document some limits that matter to you then please post to the bitpim-user mailing list and we'll endeavour to add it to the documentation.
The bigger problem is that there is a finite amount of filesystem space and that space is used by phonebook entries, SMS messages, wallpapers, call history, PRLs and just about everything else. Your limit would be the combination of the two - whichever is hit first.
I have not been able to find a command that will list the amount of free filesystem space. Consequently there is no way of knowing if you are about to hit that limit. That is why the existing documentation is vague and just tells you to be careful. In the few trials I have done filling up the filesystem the phone stops responding after the write that fills it, making it hard to recover programmatically.
There is a command to find out the size of the filesystem. The obvious algorithm would then be to sum the size of all the existing files and subtract. For every phone I tried, the existing files were larger than the filesystem size! Presumably this is because it does some scheme like in Windows CE where some files are actually in ROM and presented as being present in writable space. I even checked out the BREW developer forums, and the advice there was to create a file and fill up the filesystem and then you would know how much space there was. Unfortunately that won't work over the cable.
Here are the sizes of the embedded filesystem for some phones. Note that there are already many files stored there.
BitPim Online Help built 17 January 2010