There is no ideal, bigger pens with more cattle can be fed more efficiently, but smaller groups of cattle do better. The trick is balancing both.
A couple basics, feedlots space per head should be at least 400 sq ft. Bunk space needs to be 6 inches per animal for self-feeding (feed there all the time) and 2 feet per animal for once a day feeding.
Some big feedlots use 200 by 400 pens for 200 head, or 200 by 200 for 100 head.
Jason Trowbridge Southern Angus Farms Alberta Canada
Feedlot cattle need room or they get stressed leading to sickness, low weight gains etc.. At least 12 inches of bunk space and 350 square feet per animal is considered adequate in confinement situations, although if you can offer them more space, they will do better.