[Dovecot] Doveadm scrip-fu to determine alternate storage needs

Hello

We are using dovecot 2.0.15+mdbox+zlib and we are evaluating to buy cheap sata-based iscsi storage / sata-NFS. We want to evaluate how much space would be needed in alternate storage BEFORE buying it. I have been reading about doveadm altmove. We want to move ALL messages older than a year to alternate storage, what doveadm-script-fu could I do to identify how much space would go from a user to alternate storage?

Regards

Maria