As Mukind said, 50 GB is more than enough to install Ubuntu. As for whether or not you should use a partition bigger or smaller than that, this depends on what you're going to do with your Ubuntu system. If you install a lot of software and store a lot of files in your Ubuntu system, then you might want a bigger partition. If you are using your Ubuntu system for word processing and web browsing and storing most of your big files (e.g., videos) in your other partitions (remember, Ubuntu can access them), then you might choose to make it smaller.
But I'd go ahead and install on the 50 GB partition. It's big enough that you can put plenty of stuff in it, and small enough that your Windows partition is big enough for Windows files.
As for how to partition it, if you created it in Windows it is probably an NTFS partition. However, Ubuntu does not install on an NTFS partition--Ubuntu prefers to use ext4. Furthermore, when Ubuntu is installed, the default behavior is for it to use a much smaller, separate partition for swap. (This is called a linux-swap partition, or just a "swap partition".) Your swap partition is usually just several gigabytes in size.
Since the 50 GB partition is empty, I recommend going into Disk Management in Windows (right click Computer and click Manage, or click the Start menu, type
diskmgmt.msc and press Enter) and removing the 50 GB partition. That way, there will be 50 GB of unpartitioned space where that partition used to be, and the Ubuntu installer can automatically create both the big ext4 partition and the small linux-swap partition where the 50 GB partition used to be.
Then just boot from an Ubuntu CD/DVD or USB flash drive, install Ubuntu (you may want to try it out first on the live CD if you haven't already), tell it to install alongside existing operating systems, and before allowing it to make changes to your disk, make sure it's not shrinking down any of your remaining NTFS partitions (i.e., that it is really making its partitions in the unpartitioned space where the 50 GB partition used to be).
These are some more good resources to walk you through installing Ubuntu: