If the requirement is 2m x 2m, whether it's a client or your professor doing the asking, then you will produce a 2m x 2m poster and you will smile while you do it. ;-)
A 2m wide poster is intended for viewing at not less than 2m, probably more, so you will be more than adequate at 100 ppi (150 if there is a lot of fine detail). There's no particular reason to scale this. You can set up a 200 cm square document in Photoshop, and it will export to PDF.
[Disclaimer: If this is a class assignment, the correct answer is the one your instructor is looking for. This may or may not be the practical answer a working professional will give you, depending on the instructor. Not responsible for low grades as a result of presenting this answer as your own work.]