Looking at the headers for the ZipReadStream class, it looks like there is a method readDataOfLength. It appears that you call that method repeatedly until you have all the data from the file. I would expect a method to find out the length of the unzipped file, but did not see such a method from a quick glance.
As a side-note, I've never heard of the Objective-Zip library until a few minutes ago, and I found the 'readDataOfLength:' method in less than a minute. Perhaps you should spend some time looking through the library's header and sample code?