It would seem that anyone can legally challenge anything these days; after all that is their right.
So the answer to your question is YES.
In order to cover that possibility, your Executive Committee (E/C) merely needs to ensure that it has properly considered and properly documented its decision to limit the size of the proposed extension, and that it can justify that decision; as an example on the basis of compliance with a By-Law (preferably) such as one relating to maintaining the building’s façade.
Mine is not a legal opinion, so if you and/or other members of the E/C are concerned, a qualified legal opinion may put your mind/s at ease. One such specialist (strata) legal firm advertises on this Forum’s Home Page.
By the way, ensure that the conditions imposed include one about the responsibility for the on-going maintenance of the extension, because in the absence of such as condition that responsibility becomes one for the Owners Corporation.