Cold Iron?

In traditional medevil fantasy, cold iron negates fairy magic. Would that also apply to fairy size-change magic in Felarya?

well no ^^

To be honest Felaryan Fairies have little to do with the fairies of European myth. Felaryan fairies aren't nearly as nasty or cruel. So very cruel.

Also in many of these myths iron acts as a sort of anti magical material in general, which isn't the case on Felarya.

Actually, you can negate fairy magic with traditional iron, steel or even titanium.
Just smack the fairy on the head while she isn't looking.

successfully smacking one on the head is in regards to a successful fight against one, which is highly unlikely. fairies would more often stay away and attack at range with illusion or warp spells, or fire spells from crimson fairies, or maybe some ice ones by the mist fairies.

there are stories about fairies being cruel, but they were also spoken of as beneficial.