apple mail 4.5 max. mbox export size limited to approx. 4.7GB
I'll assume your mac is running on HFS Plus - the default file system - and in that case, the limitation is clearly the Mail application and not a file size limitation. Mbox files are just text files, so there's not much going on in them. You'll need to follow your alternative and back them up from finder or consider moving them to a Lion mac to see if this issue is corrected there.



According to the Mac OS Extended format (HFS Plus) volume and file limits  KBase article, a 2 GB file and volume limit existed from OS 10.0 to 10.1.5 and was raised to 8 GB from 10.2 to 10.2.8.

In your case on Snow Leopard (and any OS 10.4 or newer) - the file size limit is 8 exabytes. 



Stop here unless you want all sorts of silly storage calculations that don't really apply since you'd need 2.6 million 3 TB drives to make this amount of real space.

8 EB is eight million terabytes. This insane amount of storage would list price cost $480 million for the hardware alone if you ran BackBlaze storage units.