Specifies volumes sizes.
-v{Size}[b | k | m | g]
It's possible to specify several -v switches.
NOTE: Please don't use volumes (and don't copy volumes) before finishing of archiving. 7-Zip can change any volumes (including first volumes) at the end of archiving.
7z a a.7z *.txt -v10k -v15k -v2m
creates multivolume a.7z archive. First volume will be 10 KB, second will be 15 KB, and all others will be 2 MB.