The IR sensor has been described as a 1 megapixel device (1024x1024). (Some say this has been raised to 4MP.)
My understanding is that its a 4MP array. And to put this in perspective a 12MP color sensor in a consumer camera also has 4MP resolution. And the IR sensor in the AIM-9X and ASRAAM have 128 x 128 (16K) resolution.