A root partition of 10 gig is all most people will need. I have a 12 gig root, have several desktops installed, a load of apps, and the df command shows 62% used. No need for a root to be bigger, unless you have disk space to waste.
The bulk of the disk space should go to /home. This is where you will store all your stuff, pictures, videos, music files etc.
You might want to create a small swap partition. This depends on the ram you have. I have 3 gig of ram, and linux almost never swaps. A gig of swap is probably enough. So, a good basic setup is a root partition of 10 gig, a gig of swap, and the rest /home. If you install v-box, you need enough ram, it gets divided between linux, ( the host system ) and the guest ( windbloze ). I use XP on v-box, I set it up with a 1.5 gig for linux ( it will run well with less ) and 1.5 gig for XP. Some of the newer versions of windbloze need more ram. The windoze system looks like a big file, its on my /home partition.
I have used gparted, but not lately, since the installers have partitioning tools. So I use them.
gparted is not hard to use, not sure what kind of problems ( questions ) you have for it. Ask away...