You absolutely can trim the collar extra if you are referring to shortening the overall length......just trim it rounding the cut.
SportDog brand is the only collar I've owned, I've trimmed several so there was not so much extra length. Others just fold the excess over and through the collar ring.
It does appear that some models have the antenna laced in the collar material.....never really noticed that.....with that said, I've trimmed 3 and never had an issue! LOL!!