1959 Hitachi TH-666
Shirt pocket radio, thermoplastic cabinet
3 15/16 x 2 3/8 x 1 11/32 inches / 100 x 60 x 34 mm
Six transistors (Hitachi), superheterodyne circuit
One 9-volt battery
Manufactured by Hitachi Ltd., Tokyo
To be honest, I sold or traded away my Hitachi 666s long ago and I'm working from old photos here — but I do still have the "666R" cigarette lighter shown below, along with its teeny display box, both very cool!
Hitachi's claim (see the brochure images below) that the TH-666 was the smallest shirt pocket transistor radio of its time can only be true if the TH-666 had been released before the 1958 Sony TR-610, which has a slightly smaller cabinet volume than the TH-666. Alan Kastner has confirmed that the TH-666 was released after the Sony TR-610, making the brochure text's boast pure hooey, and he adds, "Hitachi would never get away with claiming that were today’s advertising laws in place at the time".
And by my own reckoning, there were likely several other radios smaller in cabinet volume that may also have pre-dated the 666, such as the Standard SR-D210, produced at the beginning of 1959.