Like Anderson said, there was a recommended size of 1MB and then the patch came along.
I presently have an OR size of 2.3MB and it seems to be working fine, just takes a bit for files to load. Having also used a smaller OR for the same files, I'd definitely suggest restricting OR size to less than 2MB at the most.
(I'm hiring Software Development Engineers in Test in Seattle, WA)
I don't know if there is an official limit, but the smaller the repository the better. Your performance will improve, and there is less chance of corruption when saving the repository.
I think the ideal size mentioned by support in the past was 1 MB.
MI recently released a patch for 8.0 that resolves corruption issues for ORs > 5MB. That seems large to me because in 6.5 I thought there was a similar issue with shared ORs > 2 MB. Hope this helps.
When you design the test scripts, you could divide the business process into logical functional pieces. Assign a separate shared OR to each logical piece. Your ORs would be relatively smaller, and might provide better performance and less maintenance problems.
How big is your OR now? And how much more, percentage-wise of the app do you need to add to it?