Well, the smallest I can think of short of a device designed specifically for cellphones would be a USB fob and an OTG adapter.
I’m not sure you could go much smaller anyway actually, as the device needs to be large enough for the antenna to have any kind of efficiency for implants - especially HF readers. And you want a cable, as something with a fixed MicroUSB plug that isn’t really, really small would easily bust the socket if you’re not super careful all the time.
That’s what I do when I want to check LF chips before and after implanting them at my piercer’s: I bring my cellphone, this teeny tiny wedge and a short OTG cable. Both the fob and the cable fit in a matchbox.