If you send me a PM to remind me tomorrow, I will go out and measure exactly. From memory though each quadrant is about 2in by 2in, and the center is about 7/8 of an inch. Please pardon me if that is incorrect, as I'm going off memory. I am quite certain though that the center disk is less than an inch in diameter.
Honestly though I'm not sure why it really matters, if you're going to make one yourself you can make it to whatever Dimensions you want to shoot at. I personally would be inclined to do 1 MOA diameter minus my pellet diameter for whatever range I would be shooting at. I should also add quickly that my quadrant Target is not as stable as I would like it to be, so if you are going to build one for yourself I would recommend welding a more stable base.