It has been confirmed that Costco will be located on Granit Drive off of Elmwood Drive. Source: City of Moncton Planning Advisory Committee. It will be larger than Saint John and Fredericton. Moncton Costco approximately 154,000 sq. ft. and Saint John location is 152,000, and the Fredericton location is 146,000.
https://www.moncton.ca/Government/Departments/Urban_Planning/Planning_Advisory_Committee__PAC_.htm