It appears from this WebmasterWorld thread that the most amount of characters you can have in your robots.txt file is 5,000. Google Sitemaps reports back an error to those who have more than 5,000 characters, stating;
Must be at most 5000 characters
The question is then, does Google ignore the full file or does it read the first 5,000 characters and ignore the remaining?
Either way, this is a good piece of information for Webmasters to know.
Forum discussion at WebmasterWorld.