Hi @marieke, and thanks for your query. Indeed the Tobii 4C tracker does depend on a connected monitor to properly work and is not compatible with a microcontroller in the manner you have specified.
There should be no problem in using a monitor of 15″.. the 27″ is just the maximum size over which we can guarantee accuracy so you don’t need to worry about ‘simulating’ a larger size.
Theoretically, you could overlay the 15″ screen itself with the stimulus you want to track.. say a book, or other 2D object.
Please bear in mind however, that the use of the Tobii 4C Eye Tracker for research or data analysis requires the purchase of a special analytical use licence to do so. You can read more about this @ https://analyticaluse.tobii.com/
In any event, if you could kindly describe in more detail your project intentions, we would be better placed to help you.