The 2GB file size limit is not Entourage's fault; it's probably a hardware limitation (I think... can someone explain why this is the case?). At least, I run into the 2GB limit with Photoshop files pretty frequently.
I'm betting that your Entourage database wasn't 2GB in size before the rebuild happened - was it?? I once did a rebuild in Entourage and it turned my ~300MB database into a 1.69GB database. It must have put a lot of extra 1s and 0s in there somehow (try compressing that file; I'll bet you get a pretty good compression ratio). I tried and tried but couldn't fix it ... good luck but I'm betting you'll have to go back to your backup. A week ago isn't bad.