Could you advise some piano scores and exercises for extreme finger stretch for a middle-size narrow (8,3 cm) hand? I found Brahms and Godowsky exercises and playing first page of the Chopin`s Etude in C for developing a better finger stretch and legato with agility. Wish to find more exercises. My left hand can almost easily play 11th (trained with guitar for 24 years), right is not trained enough to reach the edge of 11th without pressing the neighbour key. I am playing piano only for 3 years.
Firstly, I would be careful with stretching your fingers and hands too much - you don't want to cause any serious damage.
Secondly, while the ability to stretch far can be useful, pieces such as the 'Waterfall' Étude (Chopin Op. 10-1) are not about having permanently stretched hand, but rather being able to open and close your hand when necessary. This video explains it much better than I do (there's a part 2 as well).
I would recommend watching that video for a good explanation for stretching in your hands with examples.