Firegirl (the firefighter rookie you control in game) is described as a “young” rescue officer on Releases. By joining the rescue force (firefighting), she should have at least graduated highschool since a highschool level education is required to be able to join in this kind of work. We estimate that Firegirl is around 20 years old in the game.
By observing Firegirl’s height relative to the environment in this Youtube FIREGIRL: Hack ‘n Splash Rescue Gameplay Walkthrough, we created as estimated height of at least 5’0 and not more than 5’8 for Firegirl. We will use the midpoint of these two heights and that should give Firegirl and estimated height of 5 feet and 4 inches.