So, the floppy variant of OS/2 Warp 3 comes formatted in the XDF format. (from disk 2 and up)
The size of the image is 1.84MB, so it cannot be written to disks using normal disk-writing programs. (I've tried WinImage and DiskWrite, neither of them even tries to write the floppy).

I'm trying to install it onto a Pentium 100 laptop and I'm stuck on the 2nd disk, as I can't write it from Windows. Is there a tool which allows writing XDF disks to 1.44MB? (not sure if XDFCOPY can do it)

As I recall XDFCOPY is the utility used to write XDF images to floppy disks. This MUST be run on a PC with a real floppy disk controller. USB floppy drives will NOT work. Also XDFCOPY is a DOS program. It will not operate inside any Windows NT based OSes. It might work inside 3.1/95/98 but those can exit to DOS if needed anyway.

XDFCOPY can be found with PC-DOS 7 and 2k, as a DOS exe. It's a pretty standard proggie, and will write diskettes from inside 9x, NT, as well as plain DOS. I am not sure about USB floppies, though: never tried it.