It's not possible for him to have resized Jango's, or to have Jango's or Jaster's or any real mandalorian armor. Here's why. The Mandalorians of Jango's and Jaster's era had armor made from beskar, the metal mined from Mandalore, which only mandalorians know how to work into armor/weapons. Beskar as we know is darn near indestructable and can withstand a ligthsaber. Boba's armor was made from durasteel, or duraplast one of them, and was often mocked because of this, as he didn't even own a set of decent armor. In LOTF Boba buries Jango in his armor, and although one can argue that there's an off chance that Jango's armor could have been resized, you could not resize it back again to fit Jango, as you could not replace all that was removed from it in order to make it fit a child Boba. Had Boba been wearing Jango's armor there would have been no dents in the helmet or any of the armor and this website would just be called The Helmet. It's a nice story, but evidence and following events just disprove and retcon Boba wearing Jango's armor.