What is the detector size of LANDSAT 8's detector array? According to the suggested tags, this must be the right forum:
I found  that for each band of Landsat 8 over 7000 detectors are on the array. Does anyone know how big each of the sensors is?
The answer can be deduced from the main landsat 8 page, where you find the following picture of  QWIP detector arrays, and QWIP is short for Quantum well infrared photodetector, I added a gray arrow pointing to one of the 3 arrays.

I cannot give you the exact dimensions in mm, but it is clear from the SMD chip elements (SMD = surface mounted device) - one of them I highlighted with a gray ellipse, that each sensor is probably coin-sized, if not smaller. Or in numbers: I guess a side length $1.5\pm1 {\rm cm}$ seems reasonable, it depends a bit whether the sensor has the same size as the silver square or whether there is a border around.
References

Some introduction to SMD resistors