A Plus: Ours is about two acres and we have 4 in there right now. Big enough for them to get plenty of exercise. My buddy keeps 4 bulls in about an acre. I have another friend that keeps his bulls in runs about 36 feet wide and 100 ft long with access to an open front shed. I think that it is important that you give them enough room to get lots of exercise and it needs to be secure enough to keep them in. We run a real hot wire on the inside of the fence all the way around the lot.
I have a 0.8 acre pen that works fine for me when keeping just 1 bull during the good grass growing season. I think your answer depends on your pasture in Kansas, the time of year you plan to keep a bull in the pen, whether you want to suppliment feed or not, etc. Exercise requirements can be met rather easily with one of the long narrow types of pens.