I have an uncased, unlabeled step-down transformer which converts 240 UK voltage to 112v (I have measured this with a meter). It weighs 1.5kg, is 10 x 8 x 10.5cms including the attached 2-pin US socket. How can I determine it's maximum VA rating?
You can never be sure without the datasheet of the manufacturer.
Rule of thumb for a non-welded iron EI core: 50 VA/kg.