I originally gave Ubuntu 20 GB of my Windows space and realize that it's a lot more than it needs, especially since I only got it on Windows to explore the features. I don't want to delete Ubuntu, I just merely want to reclaim unneeded space from it.
You can resize a Wubi install. Either by creating a new smaller root.disk and then deleting the old one: https://help.ubuntu.com/community/ResizeandDuplicateWubiDisk
or you can do an inplace resize, which will shrink the current root.disk, but this needs to be done from a live environment (CD/USB) not while running wubi: https://help.ubuntu.com/community/ResizeWubiDisk (this may be your best option if you don't have enough space to duplicate your disk).
The only requirement is that you choose a new size that is big enough for your current files and also a free space buffer required for the operating system to run (and for new data/programs you may need). The script (first option) enforces a bare-minimum (requires about 500MB only), but not the direct resize. I'd recommend more than the minimum e.g. at least 3 GB free space, because if you try to upgrade a Wubi install with less it will likely fail.