The keys to the length of the backswing are:
1. Getting the upper body turned as far as you can–the arms need to stop when the body stops.
2. The wrists are fully hinged–about 90′.
3. The trail arm is only bent about 45′.
To get a feeling of the turn controlling the length–turn a club upside down–grip the head end–make a backswing with the lead arm only, making sure to keep that arm fairly straight. When the rotation stops the lead arm stops. Then add the trail arm at the top to see what that looks and feels like.
The problem folks get into is they think they have to get the club parallel to the ground at the top–to do this they pull the club past where it should stop with the trail arm, causing it to bend way too much.
When this happens you lose width in the swing, the arms lose their connection with the body and timing is compromised.
Some folks have a short swing to being with and probably need to work on this to lengthen their swing. As they get older a short swing will get even shorter.
Look at John Rahm and Tony Finau for examples of players with a short swing–both are fully turned in the upper body but their trail arm is not bent much–this creates a TON of width and makes their swings look short.
Here’s a John Rahm swing–look at how the rotation moves the arms and club and how little his trail arm bends at the top.