I understand that the metadata size is dictated by the size of the SSD and the size of the disk doesn't have anything to do with it. To test how memory utilization is affected, I created one hundred 1 GB dm-cache targets using 128 KB cache block size. I calculated the size of the metadata according to
https://www.redhat.com/archives/dm-devel/2012-December/msg00046.html, which is 4.12 MB per dm-cache target. Memory utilization did increase but not as much as I expected: it increased by 183 MB and not by 4.12*100 which would be the case if all metadata were kept in memory. (I didn't do any I/O to the dm-cache targets.)