Re: Larger than default 30GB needed for WUBI.
Congrats on figuring out how to resize Wubi ... but ... a couple of points to consider:
1) There is no need to restore the MBR when getting rid of Wubi because it does NOT modify the MBR in the first place! Wubi modifies the Windows BCD to add an entry for Ubuntu, not the MBR. If the entry is not removed when you uninstall Ubuntu, there are Windows GUI-based tools that can do that easily.
2) Wubi has been discontinued -- starting with 13.04. So, you're comitting yourself to a product that is already being killed off.
Ubuntu 17.04 Mate, Mint 18.1 Mate; MS Win 8.1, MS Win10 Pro.
Will not respond to PM requests for support -- use the forums.