Most desktop users today don't bother splitting
/usr off into a separate partition. In fact, Ubuntu's default setup doesn't split off any separate filesystem partitions (although Ubuntu does create a swap partition). This has the advantage of simplicity, but IMHO it's best to split off
/home to its own partition, because that makes it easier to re-install without losing your user settings and files. Beyond that, some users do split off
/var, and other partitions; but unless you know what you're doing and how to size these partitions properly, it's best not to do so.
In your case, IMHO you should delete all your Linux partitions (backing up any user data first) and re-install. Make your root (
/) partition about 20-30GiB, make a swap partition that's about 1.5-2 times the size of your RAM, and give the rest of your available space to
/home. That will put most of your free space where it will be useful to you for storing user files (in
/home), while still giving you room for installing new programs (on
/), should you decide to do so. Note that this is just my personal recommendation. Other people may give other suggestions. If so, no one of us is right or wrong in any absolute sense. There are a lot of ways to do things in Linux, and partitioning suggestions tend to vary in fairly subtle effects -- or at least, reasonable partitioning suggestions vary subtly. When a partitioning scheme is way off (as yours is, I'm afraid), the effects are more noticeably undesirable.