If you're going to play around on your computer, use heavy dependencies, if you're going to build ports, or use jails, install a lot, have lots of files, or for typical use, a 16 GB harddrive wouldn't be enough. A 16 GB harddisk would fit a very minimal install, maybe just for a command line environment. For an otherwise light windowmanager with email, internet browser and desktop applications, that won't be enough.
You can have two hardrives, use the 16GB harddisk for root, /var and swap. If you're careful, you could put /usr on it, but an additional harddisk would be needed to mount /usr/local, and /usr/home. If that swap won't be enough, an additional swap partition can be put on the second harddrive.
For a beginner question, 16GB won't be enough for everything needed for most uses.
On mine, I have a 120GB SSD hardisk for /, /usr, /usr/local, and /var. I have another SSD hardisk that's about twice as big as that for mounting: home, a jail directory, a file directory and a custom executable directory. I would still reconfigure the partition spaces, after learning about it from using my desktop on this.
My root directory is about 10GB, but I only did that for future planning, if I ever decided to install from a DVD while having 4GB of redundant space, or if I forget to mount a partition on install. If I mount the additional partitions correctly on install, I would only need about 800MB. By using other correct mounted partitions, this should be 800MB to 1GB. When, I repartition it later for a future install, I'll partition it to 2GB, to have redundant room, for future planning.
It will take learning to make the best use of a 16GB harddisk. Then, use an additional harddisk for partitions for what more is needed.