If you start with heifers, you can determine their size by breeding them younger and leaving the first calf on longer. Just like with a bull, you can work them harder, younger, and they will be a smaller size than one of the same breed that is not freshened until over 24 months and then fed heavy and suckled short times their first couple of calves. An angus, or even crosses can be kept to the 900-1000 lbs and will not affect their genetics or size calf they will raise. I just bought a small herd of 16 longhorn pairs, half the calves out of a charolais bull and half out of a simi-angus bull. There is no comparison in the calves, the charolais are far better. I would never put a simi-angus or even an angus bull with the longhorns due to large number of weak rear ends and lack of a good percentage of solid color calves. Even the solid colored calves will have longhorn heads and rears.
The cattle I bought are from 900-1,000, which are big longhorns around here if they give plenty of milk. You can find some bigger longhorns if they do not give enough milk.
The first thing I will do when I get them home is cut off their horns. This will make them easier to work with, easier on the other cattle, and there will be none of them get hung up in round bale feeders the times I use them instead of unrolling hay. Longhorns are gentle and will browse when other cattle stay in the barn, breed back quckly, and have a very long, useful life.
Another breed that is around 900-1,000 lbs around here is the white parks. Gentle, gives lots of milk, great maternal traits, and breeds back quickly. Put a charolais bull with them and get a very good selling calf that grows big. I know up north, some white parks are 13-1400 lbs, but most of the ones I have are 900-1,000 because I freshen them a little small and work them hard to keep them from getting too big for my hills and rocks.