Pay attention to the hem as it’s often overlooked at the buyers’ peril and can have a big impact on your riding comfort.
The rear of the jersey should have an extended tail to cover your lower back when you’re bent over in the riding position. Don’t be afraid to try the jersey on and assume the position, it’s the only real way you’re going to find out if the length is suitable.
Jerseys with casual fits aside, most jerseys will have elastic at the hem to help keep it in place and stop wind from sneaking in. It’s also worth looking for jerseys that include a gripper all the way round the inside of the hem to stop movement when you change position on the bike.