This is a question often asked, and it is a difficult question to answer. The answer is also very subjective.
Lets assume the following.
The camera will be operated at close focus, with a standard 20-25 degree lens. Close focus in this case will be about 30cm, and this is quite close to an electrical connection, but we will assume that it is safe to work as close as this. Then we will look at 2 different systems, a 320×240 and a 640×480. These will have an Ifov of approximately 1.3mRad and 0.66mRad. As the systems have not been tested we must assume Mfov=5xIFov, so our MFov’s are 6.5mRad and 3.3mRad, this corresponds to a minimum measurable object size of 1.95mm and 1mm respectively. In other words the 640×480 camera will be able to measure an object as small as 1mm, whereas the 320×240 can only measure objects larger than 1.95mm, and this is at close focus with the cameras. This allows you to see if the resolution is sufficient for measurement of the cables requires. Normally the smallest cable people are interested in measuring is a control cable, this would have a size of 1mm, if this is the case the higher resolution camera would be required. The lower resolution will only be useful on cables above a 2.5mm size.
These values are a close focus, the further we move away from the object, the larger the object would need to be for us to be able to measure it.
The reality is that with electrical thermography, we are unlikely to be able to work at close focus all the time, for this reason I believe that with standard lenses a 640×480 imager is the minimum resolution camera suitable for this application.