Two devices: Sensor and Interface. When interface device is close to sensor device and button is pressed, interface device produces electromagnetic field, to power up sensor device. Sensor device reads the temperature of the sensor and transmits reading to interface device using NFC technology.
Question 1 - is this possible?
Question 2 - how small can be Sensor device?
What is the purpose of the application? This strikes me as an academic question.
Purely DIY project. A hobby. I'm a Software Engineer.
I'd say it is possible. You may want to take a closer look at the method for powering up the sensor. If you're looking to use a smart device, the Tesla effect will probably drain the battery pretty quickly. I haven't looked at that since college though, so contactless power transmittal may have moved on since then.
It's like a wireless toothbrush charger, but it does not have to charge the Sensor device. Interface device have to keep Sensor device running while is placed over it.
I'm looking for a list of components that I could use for this...
And the size is a real issue. looking for a size of a SIM card or smaller...
The smallest NFC antenna I've seen is ~3mm x 3mm with an effective range of ~15mm. Other than the sensor, I think the largest part will be the controlling chipset. That might have to be a custom build if there is nothing you fancy on the market.