Acceptable to cut steak before searing?
Why would it not be acceptable to cut raw meat? It was already cut from another larger piece of meat that was cut from a whole animal. Nothing terrible will happen. A steak is not a bag of juices, nothing significant will leak off of it.

It's indeed better to cut in a smaller piece that fits your pan, the whole steak should touch the bottom of the pan otherwise you might have uneven cooking  / raw ends.