In the core rules, magic armour automatically resizes to fit the wearer.
Not any more.
In my campaigns, any magic armour remains a constant size unless it is specifically enchanted to resize to fit the wearer. This would require enlarge and reduce to be cast during the construction of the armour, in addition to any other enchantment required. This will, naturally, increase the cost and time required for the construction of the armour.
Don't lose all hope, however — I'm not totally without mercy.
Historically, custom-made (i.e. "masterwork") armour, especially plate armour, fitted the wearer very closely, and was at best uncomfortable, and at worst impossible, for others to wear. I don't intend to follow that historical precedent however, in the interests of playability.
Armours made for a specific size range will fit anyone of that size range. For example, magical armour made for a gnome (a Small humanoid) will fit any other Small humanoid (such as halflings, goblins, kobolds etc.) Armour made for a medium-sized human will also fit a medium-sized elf or orc.