When you first run WUBI it asks you how much space to allocate to it. It will default to a reasonable value based on the space available on the target partition, but you can override it if you wish. For many people this is the first look at Ubuntu and they accept the default.
Others select a windows partition (or 'drive') that has lots of space, believing that Ubuntu will be able to access this directly. It cannot. Once you've fixed the size of the install it creates a virtual disk (root.disk) that is the selected size, and will remain that way.
Of course you can store and access files on the Windows host partition, in the /host directory - but that's not always enough.
So, how to resize the virtual disk? Basically you create a new virtual disk in the size you want, and copy the old one to it. This doesn't allow you to 'add a couple of GB to your existing disk' - you need enough room for the entire new size (plus enough remaining space for Windows).
You can use this guide on ubuntuforums.org to do the resize.
My personal recommendation is not to resize, but instead to bite the bullet and migrate to a partition install. But if this is not possible, a resize is always an alternative, provided you have the space.