My thing has always been 'short reins, long arms' because my horse is fast and doesn't need to be encouraged onward 😊
This question is always so tough without having the rider FEEL how it feels to have a horse properly through the back and "on the bit". A correct frame doesn't come from your reins, it comes from the horse being engaged behind and working into the contact. I would suggest getting some lessons on a horse that frames up correctly, and once you know the feeling, you can replicate it.
To get the horse on the bit, you first need to have him in front of your legs with the right impulsion and his hindlegs engaged. A horse can't be properly on the bit if he doesn't have a good impulsion.
Then, make sure your reins are tight enough with the same length. Have a soft and constant contact with the mouth. Do some simple exercise (figure of 8, circles, serpentines, etc..) and you horse should naturally get on the bit.
For some horse it takes more time than others but be patient and you will succeed.
Good luck , hope this helps :)