I think it’s just a way to measure the Chomer Hadavar. I get this impression from any other place this is invoked.
By the way, it is quite clear from many Rashi’s and certain Gemaros that the Beis Hamikdash will be built after Moshiach comes. It doesn’t actually have to be a Stira, since Moshiach will obviously first get involved with building the Beis Hamikdash before establishing the kingdom, as the Gemara in Bava Basra says. And especially since according to Rashi it will be built instantaneously.