To figure out how much space you need to purchase, just navigate to the folder or folders you would want to backup, such as My Documents.  Right-click on the folder and then click on general and it will give you the size of the folder and folders within it. 

Most data is compressed to 1/2 the size on our servers.  If you have 1.5 gb of data, then you would need to purchase 1 gb of space.