exp username/passcode file=/dev/rmt/0M volsize=50M
where
file=/dev/rmt/0M is the tape device
once the export done upto 50M ( or anyvalue u want, that is less the size of to-be-exported file else
use volsize=0 the file shall fit in a single tape)
 

"
Be careful if you already have files on the tape, 
since [b]UNIX will start writing wherever the tape is 
positioned[/b]. You can skip to the end of the last 
file using the appropriate tape manipulation program 
(see below) and then start writing. "