You should be able to install Gparted as a gui to manage disk partitions. There are lots of other good choices, I use KDE Partition Manager, since I have KDE desktop installed.
The partition sizes, I would change. Things are a little different in linux. The root partition is where your system goes, most often not user files. I have two installs, on one system I have a 10 gig root, and the other a 20 gig root. My main system is Slackware, with the 20 gig root. It is 75 % full, and runs without problems. I have a lot of stuff installed, so you don't need 20 gig's. Keep in mind you need some space on root, since /tmp is there, and it is used by the system.
Users data goes in /home. I highly recommend this goes on a different partition to / ( root ). The reason, when you install, or upgrade your system, your user files will be preserved if you do not format /home. With your current setup, /home is on the same partition as root, so, if you upgrade, or install, everything in /home is toast.
/usr can be a different partition, if you like. I went this way on my main system. I have backups there, and my audio files there. Having it on a separate partition is the same reason as /home being separate from root.
The other thing you should create, is a swap. The size depends on the amount of ram you have. If you have a gig of ram, or more, a small swap is all you need. Some might say you don't need on at all, however with a very large disk, one doesn't take up much space, and is insurance. If you have less that a gig of ram, then you need one. Linux does not create a swap file like windoze, its a separate partition, with its own file system type. Linux will run without one, but performance may suffer, if you don't have one and are low on ram.
So, I would say 10 - 20 gig's for / ( root ).
swap a gig is overkill, you have a large disk, so no big deal.
/home as large as you want, and a separate /usr if you like. Your choice on /usr.
Hope this helps.