there are 2 answers. either you specify all file names on the command line, or you are prompted each time this is needed :

exp user/pwd ... file=exp1.dmp,exp2.dmp,...,expn.dmp filesize=1G

will export in exp1.dmp, then exp2.dmp, and so on ...

or you can do :

exp user/pwd ... file=exp.dmp filesize=1G

and you'll be prompted for the next file name when exp.dmp is full ...