Atlas mechs are like the size of the dumb ass Motorball boss I forgot the name of near the beginning of the game
Weapons are comparable in size to Highwind
Honestly, I would say they are far bigger than the Highwind itself considering the cutscene of Ultima Weapon to the Highwind side by side but it's otherwise just a worthless as fuck nitpick to what you are saying which yeah.
Though the easiest comparison to make is that Sapphire WEAPON standing up near Junon was able to fully reach the cannon and Junon as a town is fucking huge height wise overall.