There is no evidence that regular stretching execises can have any more than a marginal effect on hand span.
To quote Deahl & Wristen (2017): ‘Despite anecdotal accounts of individuals who express confidence in the value of stretching exercises or devices, there is no empirical evidence that these exercises or devices can physically extend the reach of the hand. In fact, routines or mechanical appliances of this kind have been implicated in a number of devastating injuries, most famously the case of Robert Schumann. They ought to be viewed with extreme caution and are perhaps best avoided entirely.’
Careful stretching as part of a warm-up routine before practice can. however, be of benefit to pianists.
More information: Stretching exercises – benefits, limitations and dangers
