let's say you have 2.5 Gb of data to export. you know that you could have problems with a file that is over 2 Gb, so you export specifying filesize=1G, exp will export in a first dump file of 1 Gb, when it's full, exp will continue in another 1 Gb file, and you'll end with a 500Mb file ...

and you can now import normally these export files, you just have to specify the filesize parameter in your imp command

for example :

exp system/manager file=exp.dmp log=exp.log full=y direct=y filesize=1G

imp system/manager file=exp.dmp full=y filesize=1G