It all depends on how many head your planning on feeding. Are you doing one turn a year or keeping it full? When you say confinement barn Are you talking about a deep pack building or a slatted floor? How are you going to feed? Center belt feeder, line bunks, self feeder? If line bunks will they be under the roof? You need a concrete pad for a self feeder, if that's the direction you want to go.
We have a 60x100 slatted floor building with H bunks and a conveyor style feeder. It's divided in to four pens. It's called a 275hd building, but 300- 700lb yearlings fit. We used to keep it full year round. Six to seven months is about how long they were in there. Cattle stay really clean compared to an open lot. Ten foot doors on the north and open on the south for ventilation. Five foot high concrete walls in three sides. Pipe fence on the south.
You can run into some joint issues if you put cattle on slats to small or keep them there to long.
Ours was put up in 1975, they have made some good improvements since then, but it's still better than mud caked fat cattle in March and April.