There are several factors. First of all, Microsoft doesn't recommend that you have more than 100 objects (folders, emails with or without attachments, etc.) listed directly under the Inbox of the PST. So, be careful you're not creating subfolders under the Inbox. All subfolders should be directly under the root of the PST.
Secondly, the inbox repair tool (IRT) has been known to have problems repairing PST's that are larger than 2GB. I usually don't recommend that you ever let your MAIN pst get larger than 300-400 MB. The reason is that you can have as many PST's as you want. The one that receives new mail should be the smallest and held to this restriction. If you need more space, create a new one. Once that one reaches close to a GIG in size, create another one. You get the idea!
Hope this helps...