@RayGil asked about stretching exercises for classical guitar today. He’s right there are some daunting stretches for classical guitar. I remember when I was in my first year of undergrad and my teacher had me playing something that had a G on the sixth string and B on the first strong together. I couldn’t do it, it felt huge!
I don’t think doing stretching specific exercises are necessary. And certainly note the ridiculous stretches that some seems to propose. Most stretching involve reaching with the fourth finger or the first finger, so working on getting a two fret reach between 1 to 2 and 3 to 4 helps out some.
Most of my stretching training came from playing scale patterns for jazz guitar lessons. Three of the patterns involve stretching. Top right is stretching down with the first finger and the bottom pattern is also a first finger stretch with some shifting. The pattern in the second row left side involves a fourth finger stretch.
Hand position can also help left hand stretches out. Your left wrist can rotate so the largest knuckles are parallel to the fretboard or so the fourth finger’s knuckle is further away. A more parallel position can help with some stretches, while a rotated position might help with others. Take an inventory of what your wrist is doing and experiment to find an ideal position to make a stretch easier. Then it’s just a matter of incorporating that wrist position into your movement patterns for a given piece.
In general, I guess I take a laid back approach to developing the left hand, as far as stretching goes. I think that it develops naturally as movement patterns are reinforced and more challenging pieces are practiced. For those interested in specific exercises, check out Pumping Nylon. Pages 18-20 include some left hand character builders that are well though out. All involve the first finger rather than focusing on strange stretches between 2 and 3 or 3 and 4.