This is an excellent question. Of course it does depend on how far away the object is from the camera. Camera specifications usually give a figure for FOV and IFOV, but seldom quote the MFOV.
FOV (Field of View), this refers to everything that the camera can see, as it will depend on the distance to the scene IFOV is usually given in degrees. Standard lenses are usually 20-25 degrees.
IFOV (Instantaneous Field of View), this is the size of each pixel in the camera image. It is usually given in mrad, this is also a measurement of angle. A typical figure for this would be 1.3 mrad (for a 320×240 camera, and a standard lens). What this means is that each pixel, will have a size of 1.3mm when viewing an object at a distance of 1m. Therefore at a distance of 10m the pixel size will be 13mm etc.
MFOV (Measurement Field of View), this corresponds to the smallest object that the camera can measure. MFOV is seldom quoted in camera specifications. Typically it is somewhere between 3 (3×3 pixels= 9 pixels) and 5 (5×5 pixels=25 pixels) times the IFOV. If the MFOV was 5 times the IFOV then you would need 25 pixels for measurement and the measurement area would be 1.3 mrad x5= 6.5 mrad. This would mean that the minimum measurement size would have to be 6.5mm when working at a distance of 1m, or 65 mm at a distance of 10m.