Timmer,
First, I assume you are buying 2 feeder steers - I year old in the spring, weighing 4-500 lbs? You have about 4 acres of grass for feeding and ample water via well/stream/spring on the property? Do you have trees for shade too?
Providing you have a water source, good grass growth, 4 acres can support two steers with the goal to gain 1.5 to 2 Lbs/day (400 lbs for the season) without a lot of extra corn or feed supplement. The shelter is shade, and coverage from the elements and if you have trees, water and grass, they will spend most of their time outside and not in the shelter.
With 120SF you don't have space to store feed, or hay bales unless you build a loft into the space - a higher lean to and a 4-5' loft area to store hay and grain. If you intend to grain feed them do this daily in a trough / area and store the feed elsewhere.