On a production RHEL5 server I have this:
$ du -sh /sbin /usr/sbin /bin /usr/bin /lib /usr/lib /lib64 /usr/lib64
$ du -sh /usr
I don't think you're going to find an SSD *smaller* than this, so why not just put everything on the same filesystem. Even on my Fedora desktop box (that I don't have access to right now), I don't think / and /usr are more than 15 GB together.
The split is from the era when hard drive sizes were measured in megabytes, not giga- or terabytes.